Article II, Section 2, Clause 2 of the United States Constitution, includes the Treaty Clause, which empowers the President of the United States to propose and chiefly negotiate agreements, which must be confirmed by the Senate, between the United States and other countries, which become treaties between the United States and other countries after the advice and consent of a supermajority of the United States Senate.
In the United States, the term "treaty" is used in a more restricted legal sense than in international law. U.S. law distinguishes what it calls treaties from congressional-executive agreements and sole-executive agreements. All three classes are considered treaties under international law; they are distinct only from the perspective of internal United States law. Distinctions among the three concern their method of ratification: by two-thirds of the Senate, by normal legislative process, or by the President alone, respectively. The Treaty Clause  empowers the President to make or enter into treaties with the "advice and consent" of two-thirds of the Senate. In contrast, normal legislation becomes law after approval by simple majorities in both the Senate and the House of Representatives.
Throughout U.S. history, the President has also made international "agreements" through congressional-executive agreements (CEAs) that are ratified with only a majority from both houses of Congress, or sole-executive agreements made by the President alone. Though the Constitution does not expressly provide for any alternative to the Article II treaty procedure, Article I, Section 10 of the Constitution does distinguish between treaties (which states are forbidden to make) and agreements (which states may make with the consent of Congress). The Supreme Court of the United States has considered congressional-executive and sole-executive agreements to be valid, and they have been common throughout American history. Thomas Jefferson explained that the Article II treaty procedure is not necessary when there is no long-term commitment:
It is desirable, in many instances, to exchange mutual advantages by Legislative Acts rather than by treaty: because the former, though understood to be in consideration of each other, and therefore greatly respected, yet when they become too inconvenient, can be dropped at the will of either party: whereas stipulations by treaty are forever irrevocable but by joint consent....
A further distinction embodied in U.S. law is between self-executing treaties, which do not require additional legislative action, and non-self-executing treaties which do require the enactment of new laws. These various distinctions of procedure and terminology do not affect the binding status of accords under international law. Nevertheless, they do have major implications under U.S. domestic law. In Missouri v. Holland, the Supreme Court ruled that the power to make treaties under the U.S. Constitution is a power separate from the other enumerated powers of the federal government, and hence the federal government can use treaties to legislate in areas which would otherwise fall within the exclusive authority of the states. By contrast, a congressional-executive agreement can only cover matters which the Constitution explicitly places within the powers of Congress and the President. Likewise, a sole-executive agreement can only cover matters within the President's authority or matters in which Congress has delegated authority to the President. For example, a treaty may prohibit states from imposing capital punishment on foreign nationals, but a congressional-executive agreement or sole-executive agreement cannot.
In general, arms control agreements are often ratified by the treaty mechanism. At the same time, trade agreements (such as the North American Free Trade Agreement and United States accession to the World Trade Organization) are generally voted on as a CEA, and such agreements typically include an explicit right to withdraw after giving sufficient written notice to the other parties. If an international commercial accord contains binding "treaty" commitments, then a two-thirds vote of the Senate may be required.
Between 1946 and 1999, the United States completed nearly 16,000 international agreements. Only 912 of those agreements were treaties, submitted to the Senate for approval as outlined in Article II of the United States Constitution. Since the Franklin Roosevelt presidency, only 6% of international accords have been completed as Article II treaties. Most of these executive agreements consist of congressional-executive agreements.
American law is that international accords become part of the body of U.S. federal law. Consequently, Congress can modify or repeal treaties by subsequent legislative action, even if this amounts to a violation of the treaty under international law. This was held, for instance, in the Head Money Cases. The most recent changes will be enforced by U.S. courts entirely independent of whether the international community still considers the old treaty obligations binding upon the U.S.
Additionally, an international accord that is inconsistent with the U.S. Constitution is void under domestic U.S. law, the same as any other federal law in conflict with the Constitution. This principle was most clearly established in the case of Reid v. Covert. The Supreme Court could rule an Article II treaty provision to be unconstitutional and void under domestic law, although it has not yet done so.
In Goldwater v. Carter, Congress challenged the constitutionality of then-president Jimmy Carter's unilateral termination of a defense treaty. The case went before the Supreme Court and was never heard; a majority of six Justices ruled that the case should be dismissed without hearing an oral argument, holding that "The issue at hand ... was essentially a political question and could not be reviewed by the court, as Congress had not issued a formal opposition." In his opinion, Justice Brennan dissented, "The issue of decision making authority must be resolved as a matter of constitutional law, not political discretion; accordingly, it falls within the competence of the courts". Presently, there is no official ruling on whether the President has the power to break a treaty without the approval of Congress, and the courts also declined to interfere when President George W. Bush unilaterally withdrew the United States from the ABM Treaty in 2002, six months after giving the required notice of intent.
Presidents have regarded the Article II treaty process as necessary where an international accord would bind a future president. For example, Theodore Roosevelt explained:
The Constitution did not explicitly give me power to bring about the necessary agreement with Santo Domingo. But the Constitution did not forbid my doing what I did. I put the agreement into effect, and I continued its execution for two years before the Senate acted; and I would have continued it until the end of my term, if necessary, without any action by Congress. But it was far preferable that there should be action by Congress, so that we might be proceeding under a treaty which was the law of the land and not merely by a direction of the Chief Executive which would lapse when that particular executive left office. I therefore did my best to get the Senate to ratify what I had done.
A sole-executive agreement can only be negotiated and entered into through the president's authority (1) in foreign policy, (2) as commander-in-chief of the armed forces, (3) from a prior act of Congress, or (4) from a prior treaty. Agreements beyond these competencies must have the approval of Congress (for congressional-executive agreements) or the Senate (for treaties).
In 1972, Congress passed legislation requiring the president to notify Congress of any executive agreements that are formed.
Although the nondelegation doctrine prevents Congress from delegating its legislative authority to the executive branch, Congress has allowed the executive to act as Congress's "agent" in trade negotiations, such as by setting tariffs, and, in the case of Trade Promotion Authority, by solely authoring the implementing legislation for trade agreements. The constitutionality of this delegation was upheld by the Supreme Court in Field v. Clark (1892).