France’s army was defeated by Mexico at the Battle of Puebla on May 5, 1862.
The battle occurred after France, Britain, and Spain sent forces to Mexico over unpaid foreign debts. Britain and Spain eventually negotiated withdrawals, but France under Napoleon III pursued a larger plan to establish influence in Mexico. French troops moved inland and met Mexican defenders at Puebla.
Mexico’s victory was important because the French army had a reputation for military strength and had recently performed successfully in Europe and elsewhere. The defeat did not permanently stop France’s intervention: French forces returned, captured Mexico City in 1863, and helped establish the Second Mexican Empire. Cinco de Mayo therefore celebrates a major defensive victory rather than the complete defeat of France.