Outraged by the XYZ Affair, the United States rescinded its treaties with France, resulting in the undeclared Quasi-War, fought entirely at sea.

The XYZ Affair was a political and diplomatic episode in 1797 and 1798, early in the presidency of John Adams, involving a confrontation between the United States and Republican France that led to the Quasi-War. The name derives from the substitution of the letters X, Y, and Z for the names of French diplomats Jean-Conrad Hottinguer (X), Pierre Bellamy (Y), and Lucien Hauteval (Z) in documents released by the Adams administration.

An undeclared war is a military conflict between two or more nations without either side issuing a formal declaration of war. The term is sometimes used to include any disagreement or conflict fought about without an official declaration. Since the United Nations' police action in the Korean War, some governments have pursued disciplinary actions and limited warfare by characterizing them as something else such as a military action or armed response.

The Quasi-War was an undeclared war from 1798 to 1800 between the United States and the French First Republic. It was fought almost entirely at sea, primarily in the Caribbean and off the East Coast of the United States, with minor actions in the Indian Ocean and Mediterranean Sea.