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Why did the French withdraw from Mexico after the American Civil War?

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Portrait of Napoleon III in a dark military uniform with gold epaulettes, a red sash, and medals, set against a deep red background. Painted by Hippolyte Flandrin. — Napoleon III Napoleon III, detail of a portrait by Hippolyte Flandrin, 1862; in the Versailles Museum.

The French withdrawal from Mexico was driven by a combination of mounting internal pressures in France and significant diplomatic intervention from the United States following the American Civil War.1

By 1865, the French military intervention had become increasingly unpopular in France. The campaign was costly in both men and money, yet it failed to yield the expected economic or political advantages, leading to intense criticism from the French press and parliament.1

Simultaneously, with the conclusion of the American Civil War, the United States was able to assert its influence more forcefully. U.S. Secretary of State William H. Seward applied significant diplomatic pressure on Napoleon III, and the U.S. government massed troops along the Rio Grande to demand a French exit. Facing these pressures, Napoleon III ultimately agreed to withdraw his forces, citing the failure of Maximilian to establish a stable government as justification for ending French military and financial support.12