Ongoing U.S. military operations against the Islamic State (which formerly referred to itself as the Islamic State of Iraq and the Levant, and is also commonly referred to as IS, ISIS, or ISIL) raise issues concerning the allocation of war powers between Congress and the President, including whether such operations have been (or are required to be) authorized by an act of Congress. In August 2014, President Obama ordered U.S. forces to commence airstrikes against IS targets in Iraq to assist the Iraqi government in combating the insurgent force, protect U.S. military and nonmilitary personnel in Iraq, and support certain humanitarian operations. On September 10, 2014, President Obama announced the pursuit of a strategy to "degrade and ultimately destroy" the Islamic State, including through the possible initiation of U.S. airstrikes against IS forces located in neighboring Syria. On September 23, 2014, the United States began airstrikes in Syria targeting IS forces and certain other groups within that country believed to be affiliated with Al Qaeda. The capture of significant portions of Iraqi territory in June 2014 by the Islamic State prompted significant military operations by U.S. forces in Iraq for the first time in more than three years. Between March 2003 and the end of 2011, the U.S. military forces had been deployed in Iraq first to remove the Saddam Hussein regime from power and then to assist the nascent post-Saddam government in responding to threats to the country's stability. Following the expiration of the U.S.-Iraq Security Agreement at the end of 2011, offensive U.S. military operations in Iraq were terminated and most U.S. forces were withdrawn from the country. In the wake of the successful offensive in northern and central Iraq by IS forces in June 2014, however, the Iraqi government requested U.S. assistance in responding to advancing IS forces, including through airstrikes. Shortly thereafter, the United States deployed military personnel to collect intelligence and logistical information regarding IS activities, advise Iraqi security forces, and bolster protection for U.S. personnel and facilities within Iraq. In early August, President Obama authorized U.S. forces to begin limited airstrikes against IS forces to stop further advances and to protect U.S. military and nonmilitary personnel, including those providing humanitarian assistance to Iraqi civilians trapped by IS-led forces on the Sinjar Mountain. A week later, President Obama authorized additional airstrikes to assist Iraqi forces in recapturing the Mosul Dam from IS forces. U.S. airstrikes were subsequently authorized in support of an operation to deliver humanitarian assistance to civilians located in the town of Amirli and to prevent an IS offensive against the Haditha Dam in Anbar Province. Continuing activities by the Islamic State, including the group's apparent responsibility for the execution of two U.S. journalists, led some policy makers to consider the possibility of expanding the scope of U.S. military action against the group, including potentially targeting IS forces located in Syria. In a public address on September, 10, 2014, President Obama announced the pursuit of a strategy to "degrade and ultimately destroy" the Islamic State, including through the escalation of U.S. airstrikes of IS forces in Iraq, as well as the possible initiation of U.S. airstrikes against IS forces located in neighboring Syria. On September 23, 2014, the United States began airstrikes in Syria targeting IS forces and the Khorasan Group, which is believed to be an element of Al Qaeda. Initially, the Obama Administration cited the President's authority under Article II of the Constitution as the legal basis for U.S. operations against the Islamic State. More recently, however, the Administration has claimed that the 2002 Authorization for Use of Military Force Against Iraq (2002 Iraq AUMF; P.L. 107-243 ) and the 2001 Authorization for Use of Military Force (2001 AUMF; P.L. 107-40 ) confer congressional authorization for military operations against the Islamic State. Some observers and Members of Congress have expressed disagreement with this interpretation, though it is unclear whether any dispute over application of these statutes to the Islamic State would give rise to a cognizable claim that would be resolved by the courts. As of the date of this report, Congress has not enacted legislation specifically authorizing U.S. force against the Islamic State. As part of the Continuing Appropriations Resolution, 2015 (2015 CR, P.L. 113-164 ), which was signed into law on September 19, 2014, Congress authorized the President to arm and train vetted elements of Syrian opposition groups, including for purposes of deterring attacks on the Syrian populace by the Islamic State, but the legislation expressly provides that it does not constitute statutory authorization for the introduction of U.S. forces into actual or imminent hostilities. Legislative proposals have been introduced, including H.Con.Res. 105 , passed by the House in July, and provisions within the House-passed version of the Department of Defense Appropriations Act, 2015 ( H.R. 4870 ), which appear intended to constrain the executive's ability to engage in certain types of military action in Iraq or Syria without congressional authorization. Other legislative proposals have been introduced that would expressly authorize the use of military force against the Islamic State, though in some instances these authorizations would limit the ability to deploy ground troops into combat operations, or would authorize force only for a limited period. This report addresses select legal questions raised by the use of military force against the Islamic State. Questions addressed in this report include potential sources (and limitations) of presidential authority to use military force against the Islamic State assuming the absence of express congressional authorization; the potential relevance of the 2002 Iraq AUMF, the 2001 AUMF, and the Continuing Appropriations Resolution, 2015 (2015 CR, P.L. 113-164 ); the constraints imposed by the War Powers Resolution upon U.S. military action that has not been authorized by Congress; and the applicability of the United Nations Charter to ongoing U.S. military strikes in Iraq and Syria. The report will be updated as warranted by events. Many of the legal issues discussed in this report concerning congressional oversight of U.S. military action are addressed in greater detail in CRS Report R41989, Congressional Authority to Limit Military Operations , by [author name scrubbed], [author name scrubbed], and [author name scrubbed]. Further discussion regarding U.S. operations against the Islamic State in Iraq and Syria can be found in CRS Report R43612, The "Islamic State" Crisis and U.S. Policy , by [author name scrubbed] et al.; Iraq Crisis and U.S. Policy , by [author name scrubbed] et al.; and CRS Report RL33487, Armed Conflict in Syria: Overview and U.S. Response , coordinated by [author name scrubbed]. Until recently, the executive branch cited only to the President's independent authority under Article II of the U.S. Constitution as the legal basis for the military action taken against the Islamic State. Some observers and lawmakers questioned whether some or all of these actions are lawful without congressional authorization, and some wonder whether or at what point further military action would require express congressional approval. While the Executive has recently claimed that the 2001 AUMF and 2002 Iraq AUMF provide statutory authorization for military action against the Islamic State, questions remain as to whether the President possesses independent authority to order the use of force against the group even if these statutes are deemed to be inapplicable or are subsequently repealed or narrowed by legislation. The Constitution divides authority between Congress and the President on matters of war and the use of military force. Under Article I, Section 8, Congress has the power to declare war, tax and spend for the common defense, provide for the army and navy, make rules regulating the Armed Forces, and make rules concerning captures. Additionally, Congress has authority to make all laws "necessary and proper" for carrying out not only its own powers under Article I, Section 8 but also all of the powers vested by the Constitution in the executive and judicial branches as well. On the executive side, the Constitution vests the President with the "executive Power," and appoints him "Commander in Chief of the Army and Navy of the United States," and bound to "take Care that the Laws be faithfully executed." He is also required by oath to "faithfully execute the Office of President of the United States," and, to the best of his "Ability, preserve, protect and defend the Constitution of the United States." Although not explicitly made plain by the Constitution, he is also understood to be primarily responsible for carrying out the country's foreign affairs. The circumstances when the executive may order the use of force without congressional authorization have been the subject of long-standing inter-branch and scholarly debate. The executive has long asserted that the President has independent authority to conduct at least some military operations in the absence of an authorizing act of Congress, including protecting U.S. persons abroad (and possibly foreign nationals, as well), repelling attacks against the United States or its military forces, or defending a "national interest" of the United States. In legislative enactments, however, Congress has characterized the President's independent constitutional authority to introduce U.S. forces into hostilities as being significantly more limited in scope. Notably, the War Powers Resolution of 1973 declares Congress's view that The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces. Although U.S. courts have occasionally found the exercise of military power to be impermissible when it is found to exceed or conflict with relevant statutory authority, there has been little jurisprudence concerning the scope of presidential authority to order the use of force when Congress has neither expressly authorized nor prohibited military action. A determination of the permissibility of military action that has not been authorized by Congress may require consideration of Congress's will towards the President's action and an assessment of how the Constitution allocates the asserted power between Congress and the President. When the allocation of power between the executive and legislative branches is unclear, Congress's support for, or "inertia, indifference, or quiescence" to executive action, may be relevant to an assessment of the action's constitutional validity. In Dames & Moore v. Regan , the Supreme Court assessed the legality of presidential actions which fall within the "zone of twilight in which [the President] and Congress may have concurrent authority, or in which its distribution is uncertain." In such circumstances, the Court indicated that the validity of the President's action may depend on consideration of all circumstances which shed light on the views of the legislative branch towards the action. In Dames & Moore , the petitioners challenged an executive order by President Carter to establish regulations furthering compliance with the terms of a claim-settling executive agreement with Iran. Congress had not expressly authorized these measures via statute. Nonetheless, the Court found that Congress's historical treatment of similar matters had evidenced intent to accord the President broad discretion to act on the matter: Although we have declined to conclude that [the relevant statute] directly authorizes the President's suspension of claims for the reasons noted, we cannot ignore the general tenor of Congress' legislation in this area in trying to determine whether the President is acting alone or at least with the acceptance of Congress. As we have noted, Congress cannot anticipate and legislate with regard to every possible action the President may find it necessary to take or every possible situation in which he might act. Such failure of Congress specifically to delegate authority does not, "especially ... in the areas of foreign policy and national security," imply "congressional disapproval" of action taken by the Executive. On the contrary, the enactment of legislation closely related to the question of the President's authority in a particular case which evinces legislative intent to accord the President broad discretion may be considered to "invite" "measures on independent presidential responsibility." At least this is so where there is no contrary indication of legislative intent and when, as here, there is a history of congressional acquiescence in conduct of the sort engaged in by the President. The Court "has been careful to note that past practice does not, by itself, create power." However, in at least some instances, it appears that a history of congressional acquiescence to long-standing presidential practice in a particular field "may be treated as a gloss on 'Executive Power' vested in the President." In the case of military affairs, the executive has argued that there has been a "historical pattern of presidential initiative and congressional acquiescence" to the use of military force in certain situations where statutory authorization has not been given. Accordingly, the executive may claim that congressional inaction in this realm is entitled to a greater degree of legal significance than congressional acquiescence to executive action in another field. The executive branch, while taking the view that not all military action requires congressional authorization, has acknowledged that action of a certain intensity requires the approval of Congress (at least when such action is not done to defend the country from attack, etc.). In a 2011 opinion from the Department of Justice's Office of Legal Counsel concerning the legality of military action in Libya, for example, it was alleged that [T]he historical practice of presidential military action without congressional approval precludes any suggestion that Congress's authority to declare war covers every military engagement, however limited, that the President initiates. In our view, determining whether a particular planned engagement constitutes a "war" for constitutional purposes instead requires a fact-specific assessment of the "anticipated nature, scope, and duration" of the planned military operations. This standard generally will be satisfied only by prolonged and substantial military engagements, typically involving exposure of U.S. military personnel to significant risk over a substantial period. The executive's characterization of its constitutional authority to act militarily in the absence of congressional authorization, including whether congressional inaction constitutes "acquiescence" to presidential assertions of independent authority, has been criticized by some observers and Members of Congress. However, U.S. courts have not yet found the opportunity to consider the validity of these claims, in part because challenges to military actions that have not been unauthorized by Congress have typically been dismissed on procedural grounds without the reviewing court reaching the merits of the litigants' claims. Until recently, the Obama Administration had not cited to the 2002 Iraq AUMF in public statements concerning the legal basis for military action against the Islamic State. Indeed, shortly after the deployment of U.S. military forces to Iraq to provide logistical and other support to Iraqi security forces (but prior to the commencement of U.S. airstrikes against the Islamic State), the White House reaffirmed its support for the repeal of the 2002 Iraq AUMF. However, in a congressional notification submitted on September 23, 2014, the Administration cited to the 2002 Iraq AUMF as providing statutory authorization for at least some aspects of U.S. operations against the Islamic State in Iraq and Syria. Nonetheless, some have questioned whether the 2002 Iraq AUMF could be construed to provide statutory authorization for U.S. military action against the Islamic State and other security threats presently located in Iraq. The 2002 Iraq AUMF was enacted in response to the perceived threat posed by the regime of Saddam Hussein, and in particular the prospect that Iraq had or was acquiring weapons of mass destruction. However, the operative clauses of the 2002 law do not specifically refer to the Hussein regime. Rather, the authorizing language of the 2002 Iraq AUMF permits the President to use U.S. Armed Forces in certain specified instances, including to "defend the national security of the United States against the continuing threat posed by Iraq.... " Arguably, this language could be construed to cover any threat to U.S. national security interests posed by a new crisis in Iraq, even after the removal of the Hussein regime and the establishment of a new Iraqi government. Because the 2002 Iraq AUMF does not specifically define what constitutes a "threat" to the "national security of the United States," the statute arguably affords the President a fair degree of discretion in assessing the meaning and application of those terms. It might be possible, therefore, for some to argue that the IS insurgency, by threatening to impose a new government in Iraq at odds with U.S. interests, makes Iraq once again a direct threat to U.S. security. Accordingly, it might be argued that the operative language of the 2002 Iraq AUMF confers authority on the President to use force to respond to this threat. On the other hand, the context in which the 2002 Iraq AUMF was enacted, along with language found in the act's nonoperative clauses which reference the "current Iraqi regime," could be cited in opposition to arguments that the 2002 Iraq AUMF authorizes U.S. military action over a decade after Saddam Hussein's regime had been deposed. Moreover, the withdrawal of most U.S. forces from Iraq and the cessation of offensive military activities at the end of 2011 following the termination of the U.S.-Iraq Security Agreement, along with subsequent statements by White House officials that the 2002 Iraq AUMF is not used for current U.S. activities, provide grounds for interpreting the enactment as not being relevant to U.S. military action against non-government actors operating in Iraq in 2014. If the 2002 Iraq AUMF is interpreted to cover the IS threat in Iraq, it may also become necessary to address whether or to what extent the resolution authorizes military action against IS targets in Syria. The answer to that question may be dependent on whether the conflict in Syria is deemed to be part of the same armed conflict that is occurring in Iraq and whether this can be interpreted to be a continuation of the original threat posed by Iraq. If the answer to both these questions is yes, then the 2002 Iraq AUMF, which is not expressly limited to the territory of Iraq, may be read impliedly to cover that part of the threat emanating from Syria. On the other hand, it might still be argued that the threat IS operations in Syria pose to the Assad regime is not easily construed to be a threat posed against U.S. national security "by Iraq," and that the 2002 Iraq AUMF does not confer statutory authorization of U.S. actions against IS forces located in Syria. In a notification submitted to Congress on September 23, 2014, the Obama Administration cited to the 2002 Iraq AUMF (along with the 2001 AUMF and the President's independent constitutional authority) as providing legal authority for military operations against the Islamic State in both Iraq and Syria. Recently, the Obama Administration has claimed that the 2001 AUMF provides a separate statutory ground for military action against the Islamic State. The 2001 AUMF has been the primary domestic legal authority governing U.S. military action against Al Qaeda, the Taliban, and associated forces. The application of the 2001 AUMF to the Islamic State would likely turn upon the entity's relationship with covered groups (and in particular, Al Qaeda), and whether this relationship is sufficiently close for the group to be considered an "associated force." Following the terrorist attacks of September 11, 2001, Congress passed the 2001 AUMF, which granted the President the authority: to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons. The AUMF clearly authorizes the President to use military force against entities responsible for the terrorist attacks of 9/11, as well as certain entities that harbored them. Although no organization is expressly named by the AUMF, the statute has been construed by all three branches of government to authorize military action against Al Qaeda (the entity responsible for the 9/11 attacks) and the Taliban (the entity which harbored them); as well as other "associated forces" that have fought alongside these groups in hostilities against the United States. The AUMF's application to entities other than Al Qaeda and the Taliban has been the subject of some debate, particularly regarding the nature and degree of support that would result in another organization being deemed an "associated force." The AUMF has not been construed by either the executive or judicial branches as authorizing military action against an entity solely on account of its participation in terrorism—indeed, entities that might sympathize with Al Qaeda's goals, but have no ties to the group, have been understood to fall outside the scope of the AUMF. For their part, the officials within the executive branch have claimed that to be an "associated force," a group must be both (1) an organized, armed group that has entered the fight alongside al-Qa'ida or the Taliban and (2) a co-belligerent with al-Qa'ida or the Taliban in hostilities against the United States or its coalition partners. Accordingly, an assessment of whether the 2001 AUMF confers statutory authorization for military action against the Islamic State likely depends on the nature of the group's relationship with Al Qaeda or the Taliban, and in particular whether the group is properly considered an "associated force." Although antecedent entities to the Islamic State affiliated themselves with Al Qaeda, the precise nature of the relationship between the Islamic State of Iraq and the Levant/Islamic State and Al Qaeda leaders from 2006 onward is unclear. Current IS leaders have reportedly stated that the Islamic State is not an offshoot of Al Qaeda. The Obama Administration argues that the Islamic State is covered by the AUMF because it is a successor to the version of Al Qaeda responsible for the 9/11 attacks. According to this argument, what matters is that ISIL's antecedent, Al Qaeda in Iraq, was functionally part of Al Qaeda. A schism arose between at least two components of Al Qaeda—one of which now refers to itself as the Islamic State and the other which still calls itself Al Qaeda—with both claiming the mantle of Osama Bin Laden. Accordingly, the 2001 AUMF applies equally to both groups, notwithstanding the possibility that the Islamic State does not meet the definition for an "associated force." The legitimacy of this argument appears to depend on a factual assessment of the Islamic State's relationship with the entity responsible for the 9/11 attacks, especially at the time of those attacks. If it were part of Al Qaeda prior to splitting off into a separate group, it arguably continues to bear responsibility for the attacks and falls within the plain meaning of the 2001 AUMF. Such an assessment might also consider other factors related to the Islamic State's lineage, including the composition of its membership and leadership, its association with groups that have or had been affiliated with Al Qaeda, its perception of itself as a successor to the entity responsible for the 9/11 attacks, and its mission. While appropriations measures have sometimes been interpreted to provide authority for ongoing military operations that were not otherwise expressly covered by any legislation authorizing force, the measure in the Continuing Appropriations Resolution, 2015 (2015 CR §149, P.L. 113-164 ), authorizing aid to certain Syrian rebels appears to be written in a way that forecloses such a reading. While the relevant language authorizes assistance to appropriately vetted Syrian rebel groups, the type of assistance authorized expressly includes "training, equipment, supplies, and sustainment," which could be read to imply that only similar forms of assistance are authorized. Other types of assistance are not expressly prohibited. However, section 149(i) provides that Nothing in this section shall be construed to constitute a specific statutory authorization for the introduction of United States Armed Forces into hostilities or into situations wherein hostilities are clearly indicated by the circumstances. This language seems intended to preclude reading the provision as authorizing assistance in the form of a direct combat role. On the other hand, nothing in the 2015 CR prohibits the expenditure of funds for airstrikes against IS targets in Iraq or Syria. If these operations are separately authorized by the 2001 AUMF or the 2002 Iraq AUMF, then perhaps the availability of any funds in the 2015 CR that are permitted for use in support of operations under these authorizations, coupled with knowledge of their interpretation by the executive branch, could be construed to suggest congressional acquiescence to the Executive's characterization of a connection between operations against the Islamic State and other operations which Congress had already authorized. Enacted in 1973 over President Nixon's veto, the War Powers Resolution (WPR) was an effort by Congress to reassert its role in matters of war—a role that many Members believed had been allowed to erode during the Korean and Vietnam conflicts. The WPR provides a mechanism by which Congress may ostensibly force the President to withdraw U.S. forces from those hostilities which had not been authorized by a declaration of war or specific statutory authorization. In more than four decades since its enactment, however, the mechanisms to compel troop withdrawal have never been successfully employed. Moreover, successive presidential administrations have viewed aspects of the WPR as unconstitutionally trenching upon the executive's constitutional authority in matters of war and foreign relations. Nonetheless, the WPR appears to constrain the scope and duration of military action that may be taken against IS forces in the absence of legislative authorization. The WPR requires the President to consult with Congress "in every possible instance" prior to introducing U.S. Armed Forces into actual or imminent hostilities. A report is required to be submitted to Congress within 48 hours when, absent a declaration of war, U.S. Armed Forces are introduced into "hostilities or ... situations where imminent involvement in hostilities is clearly indicated by the circumstances." Congressional notification is also required within 48 hours, absent a declaration of war, when combat-equipped U.S. forces are deployed to a foreign nation, or deployed in numbers which substantially enlarge the amount of combat-equipped U.S. forces already stationed there. Section 5(b) of the WPR provides that after this report regarding the introduction of U.S. forces into actual or imminent hostilities is submitted (or after such date that it was required to be submitted), U.S. troops must thereafter be withdrawn within 60 days (or within 90 days in certain circumstances), unless Congress authorizes continued involvement by passing a declaration of war or some other specific authorization for continued U.S. involvement in hostilities. Moreover, Section 5(c) provides a means by which Congress may, at any time, compel the withdrawal of U.S. forces from unauthorized hostilities occurring outside the United States by means of a concurrent resolution. If the Administration's assertions that the introduction of U.S. Armed Forces into hostilities against the Islamic State are authorized under previous authorizations to use force are accepted, then the WPR provision for compelling the withdrawal of U.S. forces would seem to be inapplicable. On July 28, 2014, the House agreed to a concurrent resolution ( H.Con.Res. 105 ) which provides that "The President shall not deploy or maintain United States Armed Forces in a sustained combat role in Iraq without specific statutory authorization for such use enacted after the date of the adoption of this concurrent resolution." The concurrent resolution, if agreed to by the Senate, would appear to trigger the requirements of Section 5(c) of the WPR, and arguably provide a mechanism by which Congress could compel the President to terminate any "sustained combat role in Iraq" by U.S. forces. Existing jurisprudence concerning the legally binding nature of concurrent resolutions, however, casts doubt on such a measure being interpreted to have legal effect. There appears to be a general consensus that the constitutional validity of Section 5(c) is doubtful in the aftermath of the Supreme Court's ruling in the 1983 case of INS v. Chadha . In Chadha , the Court held that for a resolution to become a law, it must go through the bicameral and presentment process in its entirety. Accordingly, concurrent or simple resolutions, which are not presented to the President for his signature, could not be used as "legislative vetoes" against executive action. Although the Chadha Court did not expressly find WPR Section 5(c) to be unconstitutional, it was listed in Justice White's dissent as one of nearly 200 legislative vetoes for which the majority had sounded the "death knell," and most commentators have agreed with this assessment. It seems highly unlikely a concurrent resolution could legally compel the termination of U.S. military operations, notwithstanding the requirements of Section 5(c) of the WPR. Nonetheless, a concurrent resolution instructing the executive branch to terminate U.S. hostilities would seem to counter any executive branch arguments that Congress had "acquiesced" to the President's use of military force. Section 5(b) of the WPR, which provides for the termination of U.S. military action within 60 days of the President submitting (or having been required to submit) a report of the introduction of U.S. forces into actual or imminent hostilities, may constrain the executive's ability to use force against the Islamic State in the absence of further action by Congress. Although some legal observers have raised constitutional concerns regarding Section 5(b), the weight of opinion, including possibly within the executive branch, appears to recognize the provision as a facially permissible exercise of Congress's constitutional authority over matters of war. Nonetheless, even assuming that the provision is constitutionally valid, it does not necessarily act as a statutory constraint to military action. As an initial matter, Section 5(b) establishes a requirement for the withdrawal of U.S. troops when they were introduced into actual or imminent hostilities without congressional authorization , unless Congress subsequently enacts legislation providing authority for the use of force or extends the deadline for the withdrawal of U.S. troops . Accordingly, if the 2002 Iraq AUMF and 2001 AUMF are construed as authorizing military action against the IS, Section 5(b) of the WPR would not establish a deadline for the termination of U.S. military strikes against IS forces. For discussion of the applicability of these statutes to military action against the Islamic State, see supra at " Does the 2002 Iraq AUMF Authorize Military Action Against the Islamic State? " and " Does the 2001 AUMF Authorize Military Action Against the Islamic State? ". Moreover, assuming for the sake of argument that the 2001 AUMF or 2002 Iraq AUMF cannot be reasonably construed to cover military action against the Islamic State, it is unclear whether this would necessarily mean that all offensive military operations against the Islamic State would be required to cease within the period established under Section 5(b) of the WPR. The executive branch has submitted numerous reports "consistent with the War Powers Resolution" since June concerning U.S. military actions related to the IS. These reports have not specified whether they are believed to satisfy the WPR's requirements relating to the introduction of U.S. forces into actual or imminent hostilities (which would trigger the 60-day timeline for the termination of U.S. action under Section 5(b) of the WPR) or are intended to satisfy the notification requirements concerning the deployment of combat-equipped U.S. forces into a foreign country (which do not trigger the termination requirements). While the deployment of U.S. forces to Iraq in June for purposes of protecting U.S. facilities and personnel located away from active zones of combat could arguably be construed as not constituting the introduction of U.S. forces into actual or imminent hostilities, this argument appears less persuasive with respect to repeated U.S. airstrikes against the Islamic State that began in August. It might be argued that each of the reports "consistent with" the WPR is intended to ensure the activities described in each report trigger distinct 60-day termination requirements, in the event that such operations cannot be construed as having been authorized by the 2001 AUMF, the 2002 Iraq AUMF, or another statute. Such an interpretation, however, would seem to enable the executive to engage in unauthorized hostilities against an enemy force for an indefinite period, so long as it promptly submitted reports to Congress describing specific military actions that had been taken (e.g., the bombing of a particular enemy facility; a battle with enemy forces at a particular location), each of which was expected to last less than 60 days. This would appear to defeat the purpose of Section 5(b) of the WPR, which was to assure the termination of unauthorized military action after a specified time period even in the absence of action by Congress. Moreover, disagreement may arise between the political branches regarding the scope of military activities covered by Section 5(b). Such a disagreement occurred during U.S. operations in support of the NATO-led mission against Muammar al Qadhafi's regime in Libya. When U.S. operations continued beyond the 60-day deadline for unauthorized hostilities established by the WPR, some argued that Section 5(b) required their immediate termination unless authorization was obtained from Congress. The Obama Administration did not challenge the constitutionality of the WPR's requirement that unauthorized hostilities be terminated within 60 days, but claimed that this requirement did not apply to ongoing U.S. operations. The continued engagement in manned and unmanned aerial attacks upon Libyan targets, in the Administration's view, was sufficiently limited so as not to constitute "hostilities" under the WPR, because they "do not involve sustained fighting or active exchanges of fire with hostile forces, nor do they involve the presence of U.S. ground troops." The use of separate WPR reports for each anti-IS operation may be intended to prevent the 60-day clock from starting at all, by suggesting that the scope and duration of each operation is too limited to amount to "hostilities" within the meaning of the WPR, even though the operations might seem more substantial in the aggregate. The Obama Administration's interpretation of the WPR's application to military activity has been criticized by some observers as overly constrained. Neither the express language of the WPR nor the legislative history of the measure provide a precise indication as to the range of military activities intended to be covered by the term "hostilities," but the legislative history suggests that Congress intended the act to apply to significant military engagements, including at least some confrontations with belligerent forces that do not involve an exchange of fire. Moreover, at least as a matter of international legal practice, repeated military strikes by one sovereign entity against another entity may be deemed "hostilities," even if such actions do not expose the attacking force to serious threat of injury. Further, some have suggested that the Administration's analysis too narrowly focuses on the question of whether the U.S. operation can be construed as "hostilities" standing alone. Some argue that in assessing whether U.S. forces are currently engaged in hostilities, it is appropriate to consider the overarching military conflict, rather than simply the supporting role that U.S. forces play within these operations. Under this interpretation, it would be appropriate to consider U.S. actions in the broader context of the ongoing military action in Iraq, including military action not only directly involving the United States, but also Iraqi security forces and the Islamic State. Ultimately, however, it seems unlikely that the dispute will be definitively resolved by the courts. Although there have been several instances where Members of Congress have brought suit against the executive and argued that a particular military action contravenes the WPR or the constitutional allocation of war powers, in all cases where final rulings have been issued, these challenges have been dismissed without the reviewing court reaching the underlying merits of the litigants' claims. The courts have variously relied on the political question doctrine, the equitable/remedial discretion doctrine, ripeness, mootness, and congressional standing concerns as grounds for dismissal. The courts have made clear, however, that while formidable, none of the aforementioned procedural barriers constitutes an insurmountable obstacle to resolving the statutory or constitutional issues concerning war powers. All of the opinions to date indicate that the barrier to the exercise of jurisdiction stems from the posture of the cases, not some institutional shortcoming. Absent such an irreconcilable conflict, however, many believe it is unlikely that the courts will venture into this politically and constitutionally charged thicket. U.S. military strikes against the Islamic State that have occurred within the territory of Iraq have been undertaken with the consent of the Iraqi government and consequently would not constitute a use of force against Iraq. However, IS fighters "have used Syria both as a staging ground for attacks in Iraq and as a parallel theater of operations," leading U.S. policy makers to expand military operations to include strikes attacking IS personnel and assets within Syria as well. Syrian officials have stated that they would perceive military attacks by the United States or other outside forces as "act[s] of aggression" if they were not undertaken with the cooperation and consent of the Syrian government. The use of force in Syria without its consent or a U.N. Security Council resolution could have implications under international law. In general, international law prohibits one state from acting militarily within the territory of another without its consent. A few exceptions to this prohibition exist, most of which are premised upon a nation's inherent right to self-defense. Prior to World War II, states were recognized as having a sovereign right to use military force against other states to vindicate any number of wrongs. In drafting the United Nations (U.N.) Charter, member states sought to reduce the incidence of war by curtailing the rights of states to use force against one another. Article 2(4) of the U.N. Charter generally prohibits member states from using or threatening to use force "against the territorial integrity or political independence of any state." In the absence of authorization from the U.N. Security Council, the sole exception to this prohibition that is expressly recognized by the U.N. Charter concerns the use of force by Member States acting in collective or individual self-defense. Specifically, Article 51 preserves the "inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security." Read literally, Article 51's articulation of the right seems to preclude a state's use of force until after an armed attack has already commenced and not merely on the threat of any use of force, but some authorities regard the right as encompassing the previously existing inherent right of self-defense under customary international law, which many likewise regard as including a right to preemptive (or "anticipatory") self-defense in the event of an imminent attack. If the state on whose territory force is to be used gives its consent, there is no violation of Article 2(4) of the UN Charter and therefore no need to invoke a theory of self-defense. Accordingly, ongoing U.S. military actions against IS forces in Iraq do not raise concerns of a violation of Iraq's territorial integrity, as these actions have been undertaken with the consent of the Iraqi government. On the other hand, military action by the United States against IS forces and Al Qaeda associated groups in Syria, absent the consent of the Syrian government (or authorization by the U.N. Security Council), would appear to conflict with the requirements of the U.N. Charter unless it could be justified as an exercise of the inherent right of individual or collective self-defense that would be permissible under Article 51. The classic formulation of the right to use force in self-defense on the territory of a foreign state was set forth by U.S. Secretary of State Daniel Webster in connection with the famous Caroline incident. In 1837 British troops attacked a private American ship, the Caroline , while it was moored for the night on the New York side of the Niagara River, asserting that the ship was being used to provide supplies to insurrectionists against British rule in Canada who were based on an island on the Canadian side of the river. The United States protested this "extraordinary outrage" and demanded an apology and reparations. In the course of the ensuing diplomatic exchanges with the British Government, Secretary of State Daniel Webster asserted that an intrusion into the territory of another state can be justified as an act of self-defense only in those "cases in which the necessity of that self-defence is instant, overwhelming, and leaving no choice of means and no moment for deliberation." Moreover, he wrote that even if justified, the use of defensive force must be proportional to the threat, "since the act, justified by the necessity of self-defence, must be limited by that necessity, and kept clearly within it." The three conditions of necessity, proportionality, and immediacy (or imminence) are widely regarded as establishing the grounds for invoking the right to resort to force extraterritorially. The application of these conditions to military action by the United States against IS forces in Syria depends, in part, upon whether such action is characterized as an act of self-defense by the United States, or is characterized as a form of collective defense undertaken on behalf of the Iraqi government or another sovereign entity (or collection of entities). Whereas the Islamic State clearly constitutes an ongoing threat to the security and territorial integrity of Iraq and Syria, the nature of the threat the Islamic State presently poses to the United States is less direct or substantial (though arguments could be made that the group currently poses a threat to U.S. persons and personnel located in Iraq). Importantly, however, if U.S. military action against IS forces outside Iraq is justified as a form of collective defense, it would appear that Iraq or any other nation that the United States sought to defend would likely need to consent to military actions that the United States took on its behalf. Another possibility for resolving a possible conflict of sovereignty in order to justify the use of force in Syria without the consent of its government would be to justify intervention on the basis that Syria is "unable or unwilling" to remove the threat emanating from its territory. In present-day application, it is not clear whether the "unwilling or unable" test is understood to be a separate test from the Caroline test, an additional consideration (for example, an element of necessity), or a substitute for one of the factors, perhaps immediacy in the case of a continuing threat. In the context of the war against Al Qaeda and the Taliban, the Obama Administration has indicated that it views as lawful military operations outside of Afghanistan without the consent of the country in which they take place if a determination has been made that the country is unable or unwilling to deal with a threat to the United States. Although the "unwilling and unable" doctrine seems to be supported by precedent in U.S. practice, it is unclear whether an international consensus exists for when or whether it may validly be applied. The United States has reportedly informed the U.N. Security Council in accordance with Article 51 of the U.N. Charter that it has commenced military operations in Syria against the Islamic State and the Khorasan Group. In its letter, the Administration explains that the operations against the Islamic State are justified as measures of both individual and collective self-defense and emphasizes that the government of Iraq had requested U.S. assistance. Moreover, the letter applies the "unable and unwilling" test, asserting that "[t]he Syrian regime has shown that it cannot and will not confront these safe-havens effectively itself." With respect to the Khorasan Group, the letter notes "the terrorist threats that they pose to the United States and our partners and allies," suggesting that military strikes of those targets are also justified as measures of individual and collective self-defense. In a subsequent statement, UN Secretary-General Ban Ki-moon seemed to accept the "unable and unwilling" theory, noting in support of the operations that "the strikes took place in areas no longer under the effective control of that Government." It remains to be seen whether the airstrikes against Syria will gain sufficient acceptance among states as a valid exercise of self-defense in order to advance the legitimacy of the "unwilling and unable" doctrine in international law.

Summary: Ongoing U.S. military operations against the Islamic State (which formerly referred to itself as the Islamic State of Iraq and the Levant, and is also commonly referred to as IS, ISIS, or ISIL) raise issues concerning the allocation of war powers between Congress and the President, including whether such operations have been (or are required to be) authorized by an act of Congress. In August 2014, President Obama ordered U.S. forces to commence airstrikes against IS targets in Iraq to assist the Iraqi government in combating the insurgent force, protect U.S. military and nonmilitary personnel in Iraq, and support certain humanitarian operations. In a public address on September 10, 2014, President Obama announced the pursuit of a strategy to "degrade and ultimately destroy" the Islamic State, including through the escalation of U.S. airstrikes against IS forces in Iraq, as well as the possible initiation of U.S. airstrikes against IS forces located in neighboring Syria. On September 23, 2014, the United States began airstrikes in Syria targeting IS forces and certain other groups within that country believed to be affiliated with Al Qaeda. As of the date of this report, Congress has not enacted legislation specifically authorizing U.S. force against the Islamic State. In enacting the Continuing Appropriations Resolution, 2015 (P.L. 113-164), which was signed into law on September 19, 2014, Congress authorized the President to arm and train vetted elements of Syrian opposition groups, including for purposes of deterring attacks on the Syrian populace by the Islamic State, but the legislation expressly provides that it does not constitute statutory authorization for the introduction of U.S. forces into actual or imminent hostilities. Initially, the Obama Administration cited the President's authority under Article II of the Constitution as the legal basis for U.S. operations against the Islamic State. More recently, however, the Administration has claimed that the 2002 Authorization for Use of Military Force Against Iraq (2002 Iraq AUMF; P.L. 107-243) and the 2001 Authorization for Use of Military Force (2001 AUMF; P.L. 107-40), provide congressional authority for military operations against the Islamic State. Some observers and Members of Congress have expressed disagreement with this interpretation, though it is unclear whether any dispute over application of these statutes to the Islamic State would give rise to a cognizable claim that would be resolved by the courts. This report addresses select legal questions raised by the use of military force against the Islamic State. Questions addressed in this report include potential sources (and limitations) of presidential authority to use military force against the Islamic State without express congressional authorization; the potential relevance of the 2002 Iraq AUMF, the 2001 AUMF, and the Continuing Appropriations Resolution, 2015; the constraints imposed by the War Powers Resolution upon U.S. military action that has not been authorized by Congress; and the applicability of the United Nations Charter to ongoing U.S. military strikes in Iraq and Syria. The report will be updated as warranted by events. Many of the legal issues discussed in this report concerning congressional oversight of U.S. military action are addressed in greater detail in CRS Report R41989, Congressional Authority to Limit Military Operations, by [author name scrubbed], [author name scrubbed], and [author name scrubbed]. Further discussion regarding U.S. operations against the Islamic State in Iraq and Syria can be found in CRS Report R43612, The "Islamic State" Crisis and U.S. Policy, by [author name scrubbed] et al.; Iraq Crisis and U.S. Policy, by [author name scrubbed] et al.; and CRS Report RL33487, Armed Conflict in Syria: Overview and U.S. Response, coordinated by [author name scrubbed].