The Islamic State (IS) organization is also referred to by its former name—the Islamic State of Iraq and the Levant (ISIL)/Islamic State of Iraq and Syria (ISIS)—and the Arabic acronym Daesh (pronounced "daash," for Dawlat al-Islamiyah f'al-Iraq wa al-Sham ), which translates to the Islamic State in Iraq and the Levant/Syria. Many observers argue that changes in Iraq's political structure as a result of the U.S.-led overthrow of Saddam Hussein helped give rise to the Islamic State. The fall of Hussein's Sunni Arab-dominated government and the ascension to power of the majority Shiite Arab population fueled deep Sunni resentment that continues today. In Syria, the Islamic State has grown in size and strength in part because of the Asad regime's use of Syria's armed forces and Iranian support to try to suppress rebellion by Syria's Sunni Arab majority. The Islamic State's direct ideological and organizational roots lie in the forces built and led by the late Abu Musab al Zarqawi in Iraq from 2002 through 2006— Tawhid wal Jihad (Monotheism and Jihad) and Al Qaeda in the Land of the Two Rivers (also known as Al Qaeda in Iraq, or AQ-I). Zarqawi took advantage of Sunni animosity toward U.S. forces and feelings of disenfranchisement at the hands of Iraq's Shiites and Kurds to advance a uniquely sectarian agenda that differed from Al Qaeda's in important ways. Some experts attribute Sunni resentment to the use by some Shiites of the democratic political process to monopolize political power in Iraq. Following Zarqawi's death at the hands of U.S. forces in June 2006, AQ-I leaders repackaged the group as a coalition called the Islamic State of Iraq (ISI). ISI lost its two top leaders in 2010 and was weakened, but not eliminated, by the time of the U.S. withdrawal in 2011. In June 2014, Islamic State leaders declared their reestablishment of the caliphate ( khilafa , lit. succession to the prophet Mohammed), dropped references to Iraq and the Levant in their name, demanded the support of believing Muslims, and named Abu Bakr al Baghdadi as caliph and imam (leader of the world's Muslims). IS leaders have highlighted Baghdadi's reported descent from the Quraysh tribe—the same tribe as the Prophet Muhammad—as well as his religious training, as qualifications for his position as caliph. The group cites its implementation of several of the historical requirements of the caliphate/imamate as further grounds for the religious legitimacy of its actions. Its Muslim critics question its legitimacy and actions. See CRS Report R43612, The "Islamic State" and U.S. Policy , by [author name scrubbed] and [author name scrubbed] for more information. Prior to 2015, the majority of terrorist attacks conducted by IS supporters were in Iraq and Syria. However, in 2015 it appears IS strategy evolved to include pursuing terrorist attacks globally. In this regard, transnational IS terrorist attacks outside of Iraq and Syria may be an instrumental tactic in a broader strategic effort to draw adversaries, including the United States, into larger-scale and more direct conflict. An example of the Islamic State's evolving strategy may be demonstrated in the numerous terrorist attacks occurring in places other than Iraq and Syria with civilian deaths rising to nearly 1,000 since January 2015. See CRS Report R43612, The "Islamic State" and U.S. Policy , by [author name scrubbed] and [author name scrubbed], and CRS Insight IN10209, European Security, Islamist Terrorism, and Returning Fighters , by [author name scrubbed] and [author name scrubbed] for more information. Since 2014, some armed groups outside of Iraq and Syria have recognized the Islamic State caliphate and pledged loyalty to Abu Bakr al Baghdadi. As of late 2015, experts consider IS adherents in Yemen, Egypt, Algeria, Saudi Arabia, Libya, Afghanistan, and Nigeria to be the most significant and capable. These groups have used the Arabic word " wilayah " (state/province) to describe themselves as constituent members of a broader IS-led caliphate. The implications of such pledges of loyalty to the Islamic State on groups' objectives, tactics, and leadership structures appear to vary and may evolve. The Obama Administration has stated that groups and individuals that are associated with the Islamic State and that participate in hostilities against the United States or its coalition partners are legitimate military targets pursuant to the 2001 Authorization for the Use of Military Force against Al Qaeda, subject to executive branch discretion. For more information, see " IS Affiliates and Adherents " in CRS Report R43612, The "Islamic State" and U.S. Policy , by [author name scrubbed] and [author name scrubbed]; CRS Insight IN10199, The Islamic State in Egypt: Implications for U.S.-Egyptian Relations , by [author name scrubbed]; " Armed Islamist Groups and Related Terrorism Threats " in CRS Report RL33142, Libya: Transition and U.S. Policy , by [author name scrubbed]; and CRS Insight IN10242, Nigeria's Boko Haram and the Islamic State , by [author name scrubbed] and [author name scrubbed]. While IS funding streams remain fluid, the group's largest revenue sources appear (based on open-source information) to include oil sales, taxation and extortion, and the sale of looted antiquities. Oil sales initially provided the majority of the group's revenue, but gradually declined as a percentage of overall IS profits due to an extensive campaign of airstrikes by the United States and coalition partners against oil and gas facilities used by the group. U.S. officials have noted that the Islamic State's financial strength depends not only on its income but also on its expenses, and the extent to which it is able to devote its resources to military operations. U.S. officials have stated that the Islamic State's decision to hold and govern territory is a financial burden for the group, and thus a vulnerability that the United States could potentially exploit by diminishing the group's ability to generate and utilize revenue. If the Islamic State cannot afford the expenses associated with governing its territory, some argue that the resulting public backlash would undermine its ability to rule. Targeting the Islamic State's finances is one of five core lines of effort to degrade and defeat the terrorist organization. General John Allen, the recently retired U.S. Special Presidential Envoy for the Global Coalition to Counter ISIL, stated in early 2015 that the United States cannot defeat the Islamic State through military efforts alone, and highlighted the need to deprive the group of access to financial resources. At present, U.S. policy focuses on disrupting IS revenue streams, limiting the group's access to formal financial systems, and imposing sanctions on the group's senior leadership and financial facilitators. The United States also has sought to collaborate with international partners, including through cooperation on financial intelligence collection and analysis. See CRS Report R43980, Islamic State Financing and U.S. Policy Approaches , by [author name scrubbed], [author name scrubbed], and [author name scrubbed] for more information. On September 10, 2014, President Obama announced the formation of a global coalition to "degrade and ultimately defeat" IS. Subsequently, some 60 nations and partner organizations agreed to participate, contributing either military forces or resources (or both) to the campaign. In Brussels in December 2014, these 60 partners agreed to organize themselves along five "lines of effort," with at least two countries in the lead for each: Supporting military operations, capacity building, and training (led by the United States and Iraq); Stopping the flow of foreign terrorist fighters (led by The Netherlands and Turkey); Cutting off IS access to financing and funding (led by Italy, the Kingdom of Saudi Arabia, and the United States); Addressing associated humanitarian relief and crises (led by Germany and the United Arab Emirates); and Exposing IS's true nature (led by the United Arab Emirates, the United Kingdom, and the United States). Coalition participation tends to be fluid, with each country contributing capabilities that are commensurate with their own national interests and comparative advantage. Since August 2015, several coalition participants have changed the roles, missions, and capabilities of the military forces they are applying to counter the Islamic State. Along with the United States, France has been at the forefront of the international coalition conducting military operations against IS in Iraq. Until September 2015, France had ruled out conducting operations in Syria—in part because it did not want to inadvertently support the Asad regime—but changed course due to growing concerns about IS. Following the November attacks, President François Hollande vowed to redouble the military campaign to destroy the Islamic State. Within 48 hours of the attacks, France launched its most aggressive air strikes yet, on the IS stronghold of Raqqa, Syria; the number of French fighter jets conducting airstrikes is to increase from 12 to 38. Hollande has also stressed that he will focus on unifying and bolstering the international military coalition fighting IS. This would include greater cooperation with the United States, Russia, and countries in the region. Russia initiated military operations in Syria in September 2015, but it did not begin robustly targeting Islamic State forces until Russian authorities concluded in mid-November that a "self-made" explosive device had brought down a Russian airliner over the Egyptian Sinai Peninsula on October 31, 2015, killing all 224 passengers on board. Statements released by the Islamic State and affiliated groups claimed responsibility for the crash, and depicted the improvised explosive devise allegedly used to carry out the attack. Russia's military operations in Syria to support the Asad regime currently appear to be independent of the global counter-IS coalition's activities. See CRS Report R44135, Coalition Contributions to Countering the Islamic State , by [author name scrubbed]; CRS Insight IN10301, France: Efforts to Counter Islamist Terrorism and The Islamic State , by [author name scrubbed]; and CRS Insight IN10360, Russian Deployments in Syria Complicate U.S. Policy , by [author name scrubbed] et al. for more information. The U.S. government continues to lead a multilateral coalition that seeks to "degrade and ultimately destroy" the Islamic State organization by progressively reducing the geographic and political space, manpower, and financial resources available to it. Stated U.S. strategy to achieve this objective consists of a number of "lines of effort," including, in partnership with several European and Arab states, direct military action, support for Iraqi and Syrian partner ground forces, intelligence gathering and sharing, and efforts to restrict flows of foreign fighters and disrupt the Islamic State's finances. Administration officials have identified areas where they believe progress has been made in implementing U.S. and allied strategy to date, but they continue to state that it may take a considerable amount of time to achieve the full range of U.S. objectives. They also note the potential for delays or setbacks. See CRS Report R43612, The "Islamic State" and U.S. Policy , by [author name scrubbed] and [author name scrubbed] for more information. High-profile terrorist attacks attributed to the Islamic State organization in several countries are altering the terms of U.S. and allied policy debates about the threat posed by the group and current strategic approaches to defeating it. At President Obama's direction, elements of the U.S. government continue to lead a multilateral coalition that seeks to "degrade and ultimately destroy" the Islamic State organization by progressively reducing the geographic and political space, manpower, and financial resources available to it. Stated U.S. strategy to achieve this objective consists of a number of "lines of effort," including, in partnership with several European and Arab states: direct military action, support for Iraqi and Syrian partner ground forces, intelligence gathering and sharing, and efforts to restrict flows of foreign fighters and disrupt the Islamic State's finances. Administration officials have identified areas where they believe progress has been made in implementing U.S. and allied strategy to date, but they continue to state that it may take a considerable amount of time to achieve the full range of U.S. objectives, while noting the potential for delays or setbacks. See CRS Report R43612, The "Islamic State" and U.S. Policy , by [author name scrubbed] and [author name scrubbed] for more information. The President in his August 2014 notifications to Congress of deployments and airstrikes in Iraq indicated his powers as Commander in Chief and Chief Executive under Article II of the Constitution gave him authority to use military force against the Islamic State. Subsequently, however, Obama Administration officials and the President's September 2014 notifications to Congress for airstrikes and other actions in Iraq and Syria stated that two enacted authorizations for use of military force (AUMFs), the Authorization for Use of Military Force (2001 AUMF; P.L. 107-40 ), and the Authorization for Use of Military Force Against Iraq Resolution of 2002 (2002 AUMF; P.L. 107-243 ), provide authorization for certain U.S. military strikes against the Islamic State in Iraq and Syria, as well as the Khorasan Group of Al Qaeda in Syria. Despite these assertions of adequate existing legislative authority, the President indicated on November 5, 2014 that he intended to enter into discussions with congressional leaders to develop a new AUMF specifically targeting the Islamic State, in order to "right-size and update whatever authorization Congress provides to suit the current fight, rather than previous fights" authorized by the 2001 and 2002 AUMFs. The President called on Congress to enact a new AUMF targeting the Islamic State in his January 2015 State of the Union address, and transmitted a draft AUMF to Congress on February 11, 2015. A number of Members of Congress have introduced legislative proposals for a new AUMF in the 113 th and 114 th Congresses, stating that existing legislation authorizing military force is outdated and insufficient when applied to the Islamic State crisis. Some Members of Congress have expressed the opinion that a new authorization is necessary to prevent the President from entering into a large-scale, long-term conflict akin to the recent conflicts in Afghanistan and Iraq. Others argue that the existing 2001 and 2002 AUMFs, authorizing the use of military force against Al Qaeda and the Taliban after the 9/11 terror attacks, and the invasion of Iraq to remove Saddam Hussein from power, respectively, are too limited in their scope and unnecessarily constrain the President's ability to defeat terrorist and extremist threats to U.S. national security and interests. These Members of Congress have in some cases introduced new authorizations that would expand the President's authority to meet growing and evolving threats, possibly applying to groups other than the Islamic State and in places other than Iraq and Syria. In many cases, Members of Congress have stressed the importance of Congress's role in the exercise of the war power in the Constitution, stating that Congress should not abdicate its responsibility to participate in the country's decision to use military force against the Islamic State. These proposals have included several types of authorizing and constraining provisions, including the following: identifying legitimate targets of military force, including the Islamic State, associated forces of the Islamic State, and others; prohibiting long-term, large-scale use of U.S. armed forces; limiting geographic area of military operations; placing time limits on existing and new authority to use military force; repealing existing AUMFs or stating that new authority supersedes older authorities with regard to the Islamic State; and requiring regular reporting, certification of certain conditions, and consultation with Congress regarding the campaign against the Islamic State. See CRS Report R43760, A New Authorization for Use of Military Force Against the Islamic State: Issues and Current Proposals in Brief , by [author name scrubbed] for more information. Al Baghdadi and other IS leaders have threatened to attack the United States since 2012. They routinely describe the United States and its non-Muslim allies as "crusaders," and encourage Islamic State supporters to attack U.S. and allied persons, facilities, and interests by any means possible overseas and at home. The group's propaganda suggests that it welcomes the prospect of direct military confrontation with the United States and U.S. partners, viewing such conflict as a harbinger of apocalyptic battles described in some Islamic religious materials. For example, in November 2014, Al Baghdadi argued that the Islamic State would continue to expand and welcomed the potential introduction of Western ground forces, saying: "soon, the Jews and Crusaders will be forced to come down to the ground and send their ground forces to their deaths and destruction, by Allah's permission." IS leaders frequently challenge the United States and others to "come down and meet us on the ground," and they view such developments as imminent and likely to end in the destruction of their enemies. Statements released by the Islamic State in the wake of the November 2015 Paris attacks contained similarly goading sentiments. In this regard, transnational IS terrorist attacks may be an instrumental tactic in a broader strategic effort to draw adversaries, including the United States, into larger-scale and more direct conflict. While statements released by the Islamic State following the November 2015 Paris attacks identified locations in Washington, DC, and New York City as potential targets for future attacks, officials and observers continue to debate the extent to which the group has the capability to direct and conduct such attacks inside the United States. U.S. officials have suggested that the individuals responsible for deadly 2015 shooting attacks in Texas and Tennessee were inspired by jihadist-Salafist propaganda, but they have not alleged any operational links between the Islamic State organization and the attackers. These U.S. attacks followed a spate of similar so-called lone wolf attacks in Europe and elsewhere, in which the alleged perpetrators appeared to be inspired by the Islamic State and/or Al Qaeda but have not necessarily been operationally linked to them or their affiliates. The Islamic State has praised these and other incidents and continues to urge supporters to conduct such attacks if they are able. In this context, U.S. officials have expressed increasing concern about the IS threat in congressional testimony and other public statements. In November 2014, National Counterterrorism Center (NCTC) Director Nicholas Rasmussen said in testimony before the Senate Select Committee on Intelligence that "the [ISIL] threat beyond the Middle East is real, although thus far limited in sophistication. However, if left unchecked, over time we can expect ISIL's capabilities to mature, and the threat to the United States homeland ultimately to increase." In October 2015, Rasmussen expressed concern about "the group's trajectory" given that it has "the ingredients that we traditionally look at as being critical to the development of an external operations capability." In the wake of the Paris, Beirut, and Sinai attacks of November 2015, and an October 2015 attack in Ankara (Turkey's capital), CIA Director John Brennan said that the Islamic State organization "has developed an external operations agenda that it is now implementing with lethal effect." He argued that the United States and its allies will have to deal with IS threats "for quite some time" and suggested that one potential motivation for the group's embrace of transnational terrorism as a tactic and strategic tool is its desire to signal continuing momentum in the face of limited progress and battlefield setbacks in Iraq and Syria since late 2014. Brennan stated his view that it is "inevitable that ISIL and other terrorist groups are going to continue to try and to attempt to carry out these attacks. That is an inevitability for at least as far as the eye can see. But to me, it's not inevitable that they're going to succeed." Efforts to prevent future attacks and assess future risks to U.S. domestic security are likely to draw from analysis and forensic study of where, how, and by whom the recent attacks were planned, organized, and directed. For more information on U.S. counterterrorism and security management , and related homeland security issues, see CRS Report R44041, Selected Issues in Homeland Security Policy for the 114th Congress , coordinated by [author name scrubbed]. There is no exact, official, and publicly available count of Americans who have been drawn to the Islamic State. The Federal Bureau of Investigation (FBI) has estimated that "upwards of 200 Americans have traveled or attempted to travel to Syria to participate in the conflict." The State Department has suggested that the Islamic State has attracted more than 22,000 foreign fighters from more than 100 countries. In a general sense, it appears that the current challenges posed by the Islamic State largely require U.S. law enforcement to identify individuals who pose a danger as terrorists and preempt their efforts to do harm. All of this draws on resources, strategies, and programs developed in response to 9/11. Analysis of U.S. counterterrorism investigations since September 11, 2001, suggests that the Islamic State (IS) and its acolytes may present broad challenges for domestic law enforcement. These challenges involve understanding and responding to a variety of terrorist actors who arguably can be sorted into five categories: The Departed—Americans, often described as foreign fighters, who plan to leave or have left the United States to fight for the Islamic State. This group includes suspects scheming to travel but who are caught before they arrive in IS territory. The Returned—American foreign fighters who trained with or fought in the ranks of the Islamic State and come back to the United States, where they can potentially plan and execute attacks at home. The Inspired—Americans lured—in part—by IS propaganda to participate in terrorist plots within the United States. The Others—Foreign IS adherents who radicalize in and originate from places outside of the United States or non-American foreign fighters active in the ranks of the Islamic State. These persons could try to enter the United States when done fighting abroad. The Lost—Unknown Americans who fight in the ranks of the Islamic State but do not plot terrorist attacks against the United States. Such individuals may come home after fighting abroad and remain unknown to U.S. law enforcement. Additionally, some American IS fighters will never book a trip back to the United States. (The post-9/11 record of U.S. counterterrorism investigations suggests this prospect. None of the Americans who have fought for al-Shabaab, a terrorist group based in Somalia, have come home to plot attacks.) Finally, some American IS supporters will perish abroad. See CRS Report R44110, The Islamic State's Acolytes and the Challenges They Pose to U.S. Law Enforcement: In Brief , by [author name scrubbed] for more information. Because modern-day terrorists and criminals are constantly developing new tools and techniques to facilitate their illicit activities, law enforcement is challenged with leveraging its tools and authorities to keep pace. For instance, interconnectivity and technological innovation have not only fostered international business and communication, they have also helped criminals carry out their operations. At times, these same technological advances have presented unique hurdles for law enforcement and officials charged with combating malicious actors. Enhanced data encryption, in part a response to privacy concerns following Edward Snowden's revelations of mass government surveillance, has opened the discussion on how this encryption could impact law enforcement investigations. Law enforcement officials have likened the new encryption to "a house that can't be searched, or a car trunk that could never be opened." There have been concerns that malicious actors, from savvy criminals to terrorists to nation states, may rely on this very encryption to help conceal their illicit activities. There is also concern that law enforcement may not be able to bypass the encryption, their investigations may be stymied, and criminals will operate above the law. Critics of these concerns contend that law enforcement maintains adequate tools and capabilities needed for their investigations. See CRS Report R44187, Encryption and Evolving Technology: Implications for U.S. Law Enforcement Investigations , by [author name scrubbed], and CRS Insight IN10400, Paris Attacks and "Going Dark": Intelligence-Related Issues to Consider , by [author name scrubbed] for more information. The layers of the Internet go far beyond the surface content that many can easily access in their daily searches. The other content is that of the Deep Web, content that has not been indexed by traditional search engines such as Google. The furthest corners of the Deep Web, segments known as the Dark Web, contain content that has been intentionally concealed. The Dark Web may be used for legitimate purposes as well as to conceal terrorism-related, criminal, or otherwise malicious activities. It is the exploitation of the Dark Web for illegal practices that has garnered the interest of officials and policymakers. Just as terrorists and criminals can rely upon the anonymity of the Dark Web, so too can the law enforcement, military, and intelligence communities. They may, for example, use it to conduct online surveillance and sting operations and to maintain anonymous tip lines. Anonymity in the Dark Web can be used to shield officials from identification and hacking by adversaries. It can also be used to conduct a clandestine or covert computer network operation such as taking down a website or a denial of service attack, or to intercept communications. Reportedly, officials are continuously working on expanding techniques to deanonymize activity on the Dark Web and identify malicious actors online. See CRS Report R44101, Dark Web , by [author name scrubbed] for more information. Iraq's sectarian and ethnic divisions—muted toward the end of the 2003-2011 U.S. military intervention in Iraq—have reemerged to fuel a major challenge to Iraq's stability and to U.S. policy in Iraq and the broader Middle East region. The resentment of Iraq's Sunni Arabs toward the Shiite-dominated central government facilitated the capture in 2014 of nearly one-third of Iraqi territory by the Sunni Islamist extremist group called the Islamic State (also known as the Islamic State of Iraq and the Levant, or ISIL). Iraq's Kurds have been separately embroiled in political and territorial disputes with Baghdad, although those differences have been subordinated to the common struggle against the Islamic State. See CRS Report RS21968, Iraq: Politics and Governance , by [author name scrubbed] and [author name scrubbed] for more information. The rise of IS and Russia's military intervention on behalf of the Syrian government have reshaped debates over U.S. policy toward the ongoing civil conflict in Syria, now in its fifth year. The Islamic State controls large areas of northeastern and central Syria, from which it continues to launch assaults on forces opposed to and aligned with the government of President Bashar al Asad. Meanwhile, fighting elsewhere pits government forces and their foreign allies against a range of anti-government insurgents, some of whom have received limited U.S. assistance. Russian military intervention in support of Asad poses a direct challenge to U.S. goals in Syria, and is raising new questions about the future of the conflict and U.S. strategy. See CRS Report RL33487, Armed Conflict in Syria: Overview and U.S. Response , coordinated by [author name scrubbed] for more information. On November 13, 2015, coordinated terrorist attacks in Paris left at least 129 people dead and over 350 injured at 6 locations throughout the city. French President François Hollande attributed the attacks to the Islamic State terrorist organization (which subsequently claimed responsibility), and asserted that France's response would be "merciless." The attacks were the worst-ever terrorist incident on French soil, and the latest in a number of examples of Islamist terrorism in France and Europe over the past year and a half. These attacks have reinforced European concerns about European citizens training and fighting with extremist groups in foreign conflicts (especially in Syria and Iraq) and heightened fears that terrorists could slip into Europe, including as part of an ongoing influx of migrants and refugees. News reports indicate that one of the assailants killed during the attacks may have entered Europe through Greece in early October with a Syrian passport as part of the refugee flows (authorities have not conclusively made this link); at least two suspects—both French nationals—may have traveled to Syria. While evidence suggests that the Islamic State was directly involved in planning and carrying out these attacks, worries also persist about "homegrown" extremists inspired by Islamist propaganda to commit violence at home without ever traveling abroad. Other recent terrorist incidents in Europe include The May 2014 killing of four people at the Jewish Museum in Brussels, Belgium; the suspect is a French Muslim who reportedly spent a year with Islamist fighters in Syria. The January 2015 attacks in Paris in which gunmen killed 17 people in three related incidents that targeted the satirical magazine Charlie Hebdo , police officers, and a kosher supermarket. The perpetrators of the attacks were French-born Muslims, with possible ties to Al Qaeda in Yemen or the Islamic State. The February 2015 shootings in Copenhagen, Denmark, in which a self-radicalized Danish-born citizen of Palestinian descent murdered two individuals—one at a cafe that had been hosting a free speech debate, another at a synagogue—and wounded five police officers. The attempted August 2015 attack on a train traveling from Amsterdam to Paris that was thwarted by six passengers, including three Americans; the suspect is a Moroccan man who may have traveled to Syria. See CRS Insight IN10209, European Security, Islamist Terrorism, and Returning Fighters , by [author name scrubbed] and [author name scrubbed]; CRS Report R44003, European Fighters in Syria and Iraq: Assessments, Responses, and Issues for the United States , coordinated by [author name scrubbed]; and CRS Insight IN10301, France: Efforts to Counter Islamist Terrorism and The Islamic State , by [author name scrubbed] for more information. The rising number of U.S. and European citizens traveling to fight with rebel and terrorist groups in Syria and Iraq has emerged as a growing concern for U.S. and European leaders, including Members of Congress. Several deadly terrorist attacks in Europe over the past year—including the killing of 17 people in Paris in January 2015—have heightened the perception that these individuals could pose a serious security threat. Increasingly, terrorist suspects in Europe appear to have spent time with groups fighting in the Middle East, especially with the Islamic State organization. Others, like the gunman who murdered two individuals in Copenhagen in February 2015, seem to have been inspired by Islamist extremist propaganda. U.S. intelligence suggests that more than 20,000 foreign fighters have traveled to the Syria-Iraq region, including at least 3,400 Westerners, since 2011. The vast majority of Western fighters are thought to be from Europe, although roughly 150 Americans have traveled or attempted to travel to Syria. U.S. authorities estimate that a handful of Americans have died in the conflict; they also assert that military operations against the Islamic State group since August 2014 have killed thousands of fighters, including an unknown number of foreigners. U.S. officials and analysts contend that the potential foreign fighter threat underscores the importance of close law enforcement ties with key European allies and existing U.S.-EU information-sharing arrangements, including those related to tracking terrorist financing and sharing airline passenger data. Some U.S. policymakers, including several Members of Congress, have expressed particular worries about European fighters in Syria and Iraq because the U.S. Visa Waiver Program (VWP) permits short-term visa-free travel to the United States for citizens of most European countries. At the same time, many point out that the VWP's existing security controls require VWP travelers to provide advanced biographic information to U.S. authorities and may help limit travel by known violent extremists. See CRS Report R44003, European Fighters in Syria and Iraq: Assessments, Responses, and Issues for the United States , coordinated by [author name scrubbed] for more information. Europe is experiencing what many consider to be its worst migration and refugee crisis since World War II, as people flee conflict and poverty in bordering regions. With the war in Syria in its fifth year, and with 4.1 million refugees in neighboring countries, more Syrians have been leaving for Europe. Other migrants and refugees originate from elsewhere in the Middle East, as well as Afghanistan, Africa, and some Western Balkans countries. Experts characterize the influxes as mixed migration, defined as flows of different groups of people—such as economic migrants, refugees, asylum-seekers, stateless persons, trafficked persons, and unaccompanied children—who travel the same routes and use the same modes of transportation (see text box). Sometimes also termed irregular migrants, these individuals do not have the required documentation, such as passports and visas, and may use smugglers and unauthorized border crossings. The surge of migrants and refugees has significantly challenged European governments and the 28-member European Union (EU), which has come under criticism for lacking coherent and effective policies. The lines of distinction between groups in the mixed migration flows have raised questions about determination of status and protection required. A key policy consideration is whether the movement is voluntary or forced. The United Nations High Commissioner for Refugees (UNHCR) asserts that 85% of those arriving in Europe by sea in 2015 are from refugee-producing countries. European governments maintain that at least some individuals seeking to enter Europe are economic migrants. See CRS In Focus IF10259, Europe's Migration and Refugee Crisis , by [author name scrubbed] and [author name scrubbed] for more information. With some European countries pledging to accept increased numbers of Syrian and other asylum seekers in the face of a refugee crisis, attention is focused on the United States and its plans to admit Syrian and other refugees in FY2016 and beyond. The Obama Administration initially proposed an overall refugee ceiling of 75,000 for FY2016 and held consultations with Congress on that proposal, as required by law. On September 20, 2015, however, Secretary of State John Kerry announced that the refugee ceiling for FY2016 would instead be 85,000. Previously the Administration had announced that the United States would admit at least 10,000 Syrian refugees in FY2016. The FY2015 worldwide refugee ceiling is 70,000 and the allocation for the Near East/South Asia region, which includes Syria, is 31,000. The FY2015 refugee admissions proposal included a discussion of U.S. plans to resettle Syrian refugees. From October 1, 2010, through August 31, 2015, the United States has admitted a total of 1,494 Syrian refugees, almost 1,300 of that total since October 1, 2014. See CRS Insight IN10355, Syrian Refugee Admissions to the United States , by [author name scrubbed], and CRS Report R44277, Syrian Refugee Admissions and Resettlement in the United States: In Brief , by [author name scrubbed] for more information. A refugee is a person fleeing his or her country because of persecution or a well-founded fear of persecution on account of race, religion, nationality, membership in a particular social group, or political opinion. Typically, the annual number of refugees that can be admitted into the United States, known as the refugee ceiling, and the allocation of these numbers by region are set by the President after consultation with Congress at the start of each fiscal year. For FY2015, the worldwide refugee ceiling is 70,000, with 68,000 admissions numbers allocated among the regions of the world and 2,000 numbers comprising an unallocated reserve. An unallocated reserve is to be used if, and where, a need develops for refugee slots in excess of the allocated numbers. The FY2015 regional allocations are as follows: Africa (17,000), East Asia (13,000), Europe and Central Asia (1,000), Latin America/Caribbean (4,000), and Near East/South Asia (33,000). Overseas processing of refugees is conducted through a system of three priorities for admission. Priority 1 comprises cases involving persons facing compelling security concerns. Priority 2 comprises cases involving persons from specific groups of special humanitarian concern to the United States (e.g., Iranian religious minorities). Priority 3 comprises family reunification cases involving close relatives of persons admitted as refugees or granted asylum. For more information on U.S. immigration inspections, the visa waiver program, and related border security issues, see CRS Report R44041, Selected Issues in Homeland Security Policy for the 114th Congress , coordinated by [author name scrubbed], and CRS Report RL31269, Refugee Admissions and Resettlement Policy , by [author name scrubbed]. The terrorist attacks in Paris last week, for which the Islamic State has claimed responsibility, have renewed concerns about terrorist travel. Following reports that at least one of the perpetrators of the attacks was carrying a Syrian passport, there has been heightened scrutiny and debate concerning the resettlement of refugees from war-torn Syria to Europe and the United States. Some of the tools the federal government employs to prevent individuals from traveling to, from, or within the United States to commit acts of terrorism include Terrorist Databases and Screening, No-Fly List and Selectee List, Criminal Sanctions, Passport Restrictions on Travel to Specific Countries, and Immigration restrictions. In some cases, the application of these tools may depend on different factors, including whether the suspected terrorist is a U.S. or foreign national. See CRS Legal Sidebar WSLG1438, Legal Tools to Deter Travel by Suspected Terrorists: A Brief Primer , by [author name scrubbed] and [author name scrubbed], and CRS Report R42336, Terrorist Watch List Screening and Background Checks for Firearms , by [author name scrubbed] for more information. In order to protect national security, the government maintains various terrorist watchlists, including the "No Fly" list, which contains the names of individuals to be denied boarding on commercial airline flights. Travelers on the No Fly list are not permitted to board an American airline or any flight on a foreign air carrier that lands or departs from U.S. territory or flies over U.S. airspace. Some persons have claimed that their alleged placement on the list was the result of an erroneous determination by the government that they posed a national security threat. In some cases, it has been reported that persons have been prevented from boarding an aircraft because they were mistakenly believed to be on the No Fly list, sometimes on account of having a name similar to another person who was actually on the list. As a result, various legal challenges to placement on the list have been brought in court. The Due Process Clause provides that no person shall be "deprived of life, liberty, or property, without due process of law." Accordingly, when a person has been deprived of a constitutionally protected liberty interest, the government must follow certain procedures. Several courts have found that placement on the No Fly list may impair constitutionally protected interests, including the right to travel internationally, and the government's redress procedures must therefore satisfy due process. Typically, due process requires that the government provide a person with notice of the deprivation and an opportunity to be heard before a neutral party. However, the requirements of due process are not fixed, and can vary according to relevant factors. When determining the proper procedural protections in a given situation, courts employ the balancing test articulated by the Supreme Court in Matthews v. Eldridge, which weighs the private interests affected against the government's interest. Courts applying this balancing test might consider several factors, including the severity of the deprivation involved in placement on the No Fly list. In addition, courts may examine the risk of an erroneous deprivation under the current procedural framework and the potential value of imposing additional procedures on the process. Finally, courts may inquire into the government's interest in preserving the status quo, including the danger of permitting plaintiffs to access sensitive national security information. See CRS Report R43730, Terrorist Databases and the No Fly List: Procedural Due Process and Hurdles to Litigation , by [author name scrubbed] for more information. Responding to reports that one individual involved in the Paris attacks was carrying a Syrian passport—which subsequent reports indicate may have been fake or stolen—a number of governors have recently expressed an intention to restrict the resettlement of Syrian refugees within their states. These announcements have prompted questions about states' authority in the refugee resettlement process and, particularly, whether a state concerned about the resettlement of Syrian refugees within its jurisdiction may take action to forestall or prevent such resettlement. It is not always clear from a governor's statements what he or she means when saying, for example, that a state "will temporarily suspend accepting new Syrian refugees." However, states would appear to have some discretion as to the terms on which state agencies participate in the federally funded refugee resettlement program, although a state likely could not opt to participate actively in the resettlement of refugees from some countries but not others. In contrast, a state lacks the power to prohibit a Syrian refugee admitted into the United States from physically entering or remaining within the state's jurisdiction. See CRS Legal Sidebar WSLG1440, Can States and Localities Bar the Resettlement of Syrian Refugees Within Their Jurisdictions? , by [author name scrubbed] and [author name scrubbed] for more information. The following table provides names and contact information for selected CRS experts on related foreign policy, defense, legal, U.S. domestic policy, and other associated legislative issues.

Summary: In the wake of the deadly November 13, 2015, terrorist attacks in Paris, U.S. policymakers are faced with a wide range of strategy and operational considerations related to the activities of and threats emanating from the Islamic State (IS). A terrorist attack such as this prompts an examination of U.S. domestic security precautions; the role of allies and coalition partners; the appropriate military and diplomatic reactions; the safety and security of infrastructure and that of travelers; and numerous additional discrete issues that require the active involvement of dozens of federal, state, and local government agencies. With the attacks in Beirut, over Egypt, and in Paris, the Islamic State has demonstrated a transnational capability that suggests its strategic objectives and tactics have evolved, gaining strength in some areas and losing capacity in others. The response to these attacks by the United States and other nations continues to evolve as the threat posed by IS changes. This report poses frequently asked questions with answers excerpted from other CRS products. Each section contains references to the full reports in which the material appears. This report will be updated as additional products become available and events warrant.