The United States is a party to numerous security agreements with other nations. The topics covered, along with the significance of the obligations imposed upon agreement parties, may vary. Some international security agreements entered by the United States, such as those obliging parties to come to the defense of another in the event of an attack, involve substantial commitments and have traditionally been entered as treaties, ratified with the advice and consent of the Senate. Other agreements dealing with more technical matters, such as military basing rights or the application of a host country's laws to U.S. forces stationed within, are entered more routinely and usually take a form other than treaty. Regardless of the form of a security arrangement, Congress has several tools which enable it to exercise oversight regarding the negotiation, form, conclusion, and implementation of the agreement by the United States. This report begins by providing a general background as to the types of international agreements that are binding upon the United States, as well as considerations affecting whether they take the form of a treaty or an executive agreement. Next, the report examines historical precedents, with specific attention paid to past agreements entered with Afghanistan, Iraq, Germany, Japan, South Korea, and the Philippines. Finally, the report discusses the oversight role that Congress exercises with respect to entering and implementing international agreements involving the United States. Under U.S. law, a legally binding international agreement can be entered into pursuant to either a treaty or an executive agreement. The Constitution allocates primary responsibility for entering such agreements to the executive branch, but Congress also plays an essential role. First, in order for a treaty (but not an executive agreement) to become the "Law of the Land," the Senate must provide its advice and consent to treaty ratification by a two-thirds majority. Alternatively, Congress may authorize congressional-executive agreements. Many treaties and executive agreements are not "self-executing," meaning that in order for them to take effect domestically, implementing legislation is required to provide U.S. bodies with the authority necessary to enforce and comply with the agreements' provisions. While some executive agreements do not require congressional approval, adherence to them may nonetheless be dependent upon Congress appropriating necessary funds or authorizing the activities to be carried out (where compliance with the agreement would contravene some statutory provision). Under U.S. law, a treaty is an agreement negotiated and signed by the executive branch, which enters into force if it is approved by a two-thirds majority in the Senate and is subsequently ratified following presidential signature. The Senate may, in considering a treaty, condition its consent on certain reservations, declarations, and understandings concerning treaty application. If accepted, these reservations, declarations, and understandings may limit and/or define U.S. obligations under the treaty. The great majority of international agreements that the United States enters into are not treaties but executive agreements —agreements made by the executive branch that are not submitted to the Senate for its advice and consent. There are three types of prima facie legal executive agreements: (1) congressional-executive agreements , in which Congress has previously or retroactively authorized an international agreement entered into by the executive; (2) executive agreements made pursuant to an earlier treaty , in which the agreement is authorized by a ratified treaty; and (3) sole executive agreements , in which an agreement is made pursuant to the President's constitutional authority without further congressional authorization. The executive's authority to promulgate the agreement is different in each case. Although executive agreements are not specifically discussed in the Constitution, they nonetheless have been considered valid international compacts under Supreme Court jurisprudence and as a matter of historical practice. Starting in the World War II era, reliance on executive agreements has grown significantly, with the number of international agreements entered as executive agreements significantly dwarfing those entered as treaties. Although some have argued that certain agreements may only be entered as treaties, subject to the advice and consent of the Senate, this view has been rejected by many scholars. Adjudication of the propriety of executive agreements has been rare, in significant part because plaintiffs often cannot demonstrate that they have suffered a redressable injury giving them standing to challenge an agreement, or fail to make a justiciable claim. In 2001, the Eleventh Circuit Court of Appeals held that the issue of whether the North American Free Trade Agreement (NAFTA) was a treaty requiring approval by two-thirds of the Senate presented a nonjusticiable political question. It does not appear that an executive agreement has ever been held invalid by the courts on the grounds that it was in contravention of the Treaty Clause. Nonetheless, as a matter of historical practice, some types of agreements have been concluded as treaties, while others have been concluded as executive agreements. In the case of congressional-executive agreements, the "constitutionality ... seems well established." Unlike treaties, where only the Senate plays a role in authorization, both houses of Congress are involved in the authorizing process for congressional-executive agreements. Congressional authorization takes the form of a statute passed by a majority of both houses of Congress. Historically, congressional-executive agreements cover a wide variety of topics, ranging from postal conventions to bilateral trade to military assistance. NAFTA and the General Agreement on Tariffs and Trade (GATT) are notable examples of congressional-executive agreements. Congressional-executive agreements also may take different forms. Congress may enact legislation authorizing the executive to negotiate and enter agreements with other countries on a specific matter. A congressional-executive agreement may also take the form of a statute passed following the negotiation of an agreement which incorporates the terms or requirements of the agreement into U.S. law. Such authorization may be either explicit or implied by the terms of the congressional enactment. The legitimacy of agreements made pursuant to treaties is also well established, though controversy occasionally arises as to whether the agreement was actually imputed by the treaty in question. Since the earlier treaty is the "Law of the Land," the power to enter into an agreement required or contemplated by the treaty lies fairly clearly within the President's executive function. However, the Senate occasionally conditions its approval of a treaty upon a requirement that any subsequent agreement made pursuant to the treaty also be submitted to the Senate as a treaty. Sole executive agreements rely on neither treaty nor congressional authority for their legal basis. There are a number of provisions in the Constitution that may confer limited authority upon the President to promulgate such agreements on the basis of his power to conduct foreign affairs. The Litvinov Assignment, under which the Soviet Union purported to transfer claims against American assets previously nationalized by the Soviet Union, is an example of a sole executive agreement. If the President enters into an executive agreement pursuant to and dealing with an area where he has clear, exclusive constitutional authority—such as an agreement to recognize a particular state for diplomatic purposes—the agreement is legally permissible regardless of Congress's opinion on the matter. If, however, the President enters into an agreement and his constitutional authority over the subject matter is unclear, or if Congress also has constitutional authority over the subject matter, a reviewing court may consider Congress's position in determining whether the agreement is enforceable as U.S. law. If Congress has given implicit approval to the President to enter into the agreement, or is silent on the matter, a reviewing court might be more likely to view the agreement as valid. When Congress opposes the agreement and the President's constitutional authority to enter the agreement is ambiguous, it is unclear if or under what circumstances a court would recognize such an agreement as controlling. Because sole executive agreements do not rely on congressional authority to support their legality, they do not require congressional approval to become binding, at least as a matter of international law. Courts have recognized, however, that if a sole executive agreement conflicts with preexisting federal law, the earlier law will remain controlling in most circumstances. Even if a sole executive agreement does not conflict with prior federal law, Congress may still act to limit the agreement's effect through a subsequent legislative enactment, so long as it has constitutional authority to regulate the matter covered by the agreement. In the security context, Congress has clear constitutional authority to enact measures that would limit the effect of sole executive agreements involving military commitments. Article I, Section 8 of the Constitution accords Congress the power "To lay and collect Taxes ... to ... pay the Debts and provide for the common Defence," "To declare War, grant letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water," "To raise and support Armies," "To provide and maintain a Navy," "To make Rules for the Government and Regulation of the land and naval Forces," as well as "To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions" and "To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States." Further, Congress is empowered "To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers" as well as "all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof." In addition to the constitutional provisions that provide Congress with authority to legislate on matters concerning military affairs, Congress also has virtual plenary power over appropriations—authority not qualified with reference to Congress's enumerated powers under Article I, Section 8. The Appropriations Clause provides that "[n]o money can be paid out of the Treasury unless it has been appropriated by an act of Congress." Accordingly, adherence to pledges made in sole executive agreements may be dependent upon the availability of appropriations authorized by Congress. Congress may specify the terms and conditions under which appropriations may be used, so long as it does not impose unconstitutional conditions upon the use of appropriated funds. Not every pledge, assurance, or arrangement made between the United States and a foreign party constitutes a legally binding international agreement.