XYZ Affair, U.S. diplomatic incident involving a commission sent to France in 1797 to negotiate outstanding differences between the two countries. These differences arose largely out of the refusal by the United States to come to the aid of France, then at war with Great Britain, as stipulated in the Franco-American treaty of 1778. Further, under the terms of Jay's Treaty, which the U.S. and Great Britain concluded in 1794, the U.S. had accepted the British view of the rights and obligations of neutrals and subsequently had ordered French ships out of American ports. The French retaliated by preying upon American shipping. In an attempt to reach a settlement, President John Adams appointed a commission consisting of the American statesmen John Marshall, Charles Cotesworth Pinckney, and Elbridge Thomas Gerry to negotiate with the French government. The commission arrived in Paris in October 1797.

The French foreign minister Charles Maurice de Talleyrand-Périgord informed the commission through three secret agents that the Directory (the French executive) would not negotiate with the mission unless the U.S. agreed to “lend” the French government the equivalent of $10 million and to present him, Talleyrand, with a “gift” of $250,000. The commissioners rejected the proposals and forwarded the substance of the French demands to Adams, who submitted them to Congress. During the negotiations with the French agents, Pinckney is said to have uttered the now famous slogan “millions for defense, but not one cent for tribute.”

In April 1798, the dispatches of the American commissioners were made public; in the otherwise complete copies, the letters X, Y, and Z were substituted for the names of Talleyrand's emissaries. The exposure of French diplomatic tactics engendered widespread indignation, and Congress authorized preparations for war. Although U.S. and French naval vessels clashed on a number of occasions, war was not formally declared. The French government adopted a conciliatory attitude and later in 1798 officially repudiated the so-called X, Y, and Z agents, denouncing them as charlatans. Adams, who also wished to avoid war, sent a new commission to France late in 1799. Napoleon Bonaparte overthrew the Directory and established himself as France's leader before the commissioners reached Paris. In September 1800, they finally arranged with him a convention that once more put the two nations on friendly terms.