The Senate's Role in Treaties
The Constitution provides that the president "shall have Power,
by and with the Advice and Consent of the Senate, to make Treaties,
provided two-thirds of the Senators present concur" (Article II, section 2). The Constitution's framers gave the Senate a share of the treaty power in order to give the president the benefit of the Senate's advice and counsel, check presidential power, and safeguard the sovereignty of the states by giving each state an equal vote in the treatymaking process. As Alexander Hamilton explained in Federalist no. 75, “the operation of treaties as laws, plead strongly for the participation of the whole or a portion of the legislative body in the office of making them.” The constitutional requirement that the Senate approve a treaty with a two-thirds vote means that successful treaties must gain support that overcomes partisan division. The two-thirds requirement adds to the burdens of the Senate leadership, and may also encourage opponents of a treaty to engage in a variety of dilatory tactics in hopes of obtaining sufficient votes to ensure its defeat.
The Senate does not ratify treaties—the Senate approves or rejects a resolution of ratification. If the resolution passes, then ratification takes place when the instruments of ratification are formally exchanged between the United States and the foreign power(s).
Most treaties submitted to the Senate have received its advice and consent to ratification. During its first 200 years, the Senate approved more than 1,500 treaties and rejected only 21. A number of these, including the Treaty of Versailles, were rejected twice. Most often, the Senate has simply not voted on treaties that its leadership deemed not to have sufficient support within the Senate for approval, and in general these treaties have eventually been withdrawn. At least 85 treaties were eventually withdrawn because the Senate never took final action on them. Treaties may also remain in the Senate Foreign Relations Committee for extended periods, since treaties are not required to be resubmitted at the beginning of each new Congress. There have been instances in which treaties have lain dormant within the committee for years, even decades, without action being taken.
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Terminating Treaties
The Constitution is silent about how treaties might be terminated. The breaking off of two treaties during the Jimmy Carter administration stirred controversy. In 1978 the president terminated the U.S. defense treaty with Taiwan in order to facilitate the establishment of diplomatic relations with the People's Republic of China. Also in 1978 the new Panama Canal treaties replaced three previous treaties with Panama.
In one case, the president acted unilaterally; in the second, he terminated treaties in accordance with actions taken by Congress. Only once has Congress terminated a treaty by a joint resolution; that was a mutual defense treaty with France, from which, in 1798, Congress declared the United States "freed and exonerated." In that case, breaking the treaty almost amounted to an act of war; indeed, two days later Congress authorized hostilities against France, which were only narrowly averted.
https://www.senate.gov/artandhistory/history/common/briefing/Treaties.htm
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