The Treaty Clause of the United States Constitution (Article II, Section 2, Clause 2) establishes the procedure for ratifying international agreements. It empowers the President as the primary negotiator of agreements between the United States and other countries, and holds that the advice and consent of a two-thirds supermajority of the Senate renders a treaty binding with the force of federal law.
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Background
Treaties under the Articles of Confederation
As with the drafting of the U.S. Constitution as a whole, the Treaty Clause was influenced by perceived flaws and limitations of the Articles of Confederation, the first governmental framework of the United States. The Articles established a weak central government and accorded significant autonomy and deference to the individual states. The unicameral Congress of the Confederation was the sole national governing body, with both legislative and executive functions, including the power to make treaties. However, to take effect, treaties needed the approval of a supermajority of states (nine out of thirteen), a high bar that prevented many foreign pacts from being made. Although states were obligated by the Articles not to "interfere" with Congress' international commitments, in practice they often ignored or even defied such agreements.
The weakness of the Articles with respect to foreign affairs was most pronounced with respect to the Treaty of Paris with Great Britain, which stipulated that Congress protect the property rights of British creditors and Loyalists. Many state governments either failed to enforce the treaty or took measures to deliberately violate it. In response to protests by the British government, the U.S. Secretary for Foreign Affairs John Jay, could only propose that Congress request state legislatures to repeal all legislation that conflicted with the treaty and to authorize state courts to enforce it. While a bare majority of states complied to varying degrees, the inability of the national government to follow through on its obligations to foreign powers proved to be both an international embarrassment and potentially damaging to national interests; many Founding Fathers worried that nations would likewise renege on treaties with the United States or refrain from entering treaties that would be beneficial to trade and commerce.
Debate about treaty making in the Constitutional Convention
In response to various concerns about the ineffectualness of the Articles, a Constitutional Convention was held in 1787 to debate and draft a more robust governing document. During the Convention, it was initially contemplated that the U.S. Senate, the newly proposed upper house of Congress, would have the power to make treaties (as well as to appoint ambassadors and judges of the Supreme Court). Just ten days before the Convention adjourned, it was decided that these powers would be shared with the President. Many delegates cited the established international tradition of executives holding exclusive power over foreign relations and agreements; the participation of the Senate through the "advice and consent" mechanism was added as something of a compromise.