Cinco de Mayo (Mexican Spanish: [ˈsiŋko ðe ˈmaʝo]; Spanish for 'Fifth of May') is an annual celebration held on May 5 to celebrate Mexico's victory over the Second French Empire at the Battle of Puebla in 1862, led by General Ignacio Zaragoza. Zaragoza died months after the battle from an illness, however, and a larger French force ultimately defeated the Mexican army at the Second Battle of Puebla and then occupied Mexico City. Following the end of the American Civil War in 1865, the United States began lending money and guns to the Mexican Liberals, pushing France and Mexican Conservatives to the edge of defeat. At the opening of the French chambers in January 1866, Napoleon III announced that he would withdraw French troops from Mexico. In reply to a French request for American neutrality, the American secretary of state William H. Seward replied that French withdrawal from Mexico should be unconditional.

More popular in the United States than in Mexico, Cinco de Mayo has become associated with the celebration of Mexican-American culture. Celebrations began in Columbia, California, where they have been observed annually since 1862. The day gained nationwide popularity beyond those of Mexican-American heritage in the 1980s due to advertising campaigns by beer, wine, and tequila companies; today, Cinco de Mayo generates beer sales on par with the Super Bowl. In Mexico, the commemoration of the battle continues to be mostly ceremonial, such as through military parades or battle reenactments. The city of Puebla marks the event with various festivals and reenactments of the battle.

Cinco de Mayo is sometimes mistaken for Mexican Independence Day—the most important national holiday in Mexico—which is celebrated on September 16, commemorating the Cry of Dolores in 1810, which initiated the Mexican War of Independence from Spain. Cinco de Mayo has been referenced and featured in entertainment media, and has become an increasingly global celebration of Mexican culture, cuisine, and heritage.

Cinco de Mayo has its roots in the second French intervention in Mexico, which took place in the aftermath of the 1846–48 Mexican–American War and the 1858–61 Reform War. The Reform War was a civil war that pitted Liberals (who believed in separation of church and state and freedom of religion) against Conservatives (who favored a tight bond between the Catholic Church and the Mexican state). These wars nearly bankrupted the Mexican Treasury. On July 17, 1861, Mexican President Benito Juárez issued a moratorium in which all foreign debt payments would be suspended for two years. In response, France, Spain, and the United Kingdom held a convention in London and joined in alliance to send naval forces to Veracruz to demand reimbursement. France, at the time ruled by Napoleon III, decided to use the opportunity to establish an empire in Mexico that would favor French interests, whereupon Britain and Spain negotiated with Mexico and peacefully withdrew. The empire was part of an envisioned "Latin America" (term used to imply cultural kinship of the region with France) that would rebuild French influence in the American continent and exclude Anglophone American territories.^{[citation needed]}

Late in 1861, a well-armed French fleet attacked Veracruz, landing a large French force and driving President Juárez and his government into retreat. Moving on from Veracruz towards Mexico City, the French army encountered heavy resistance from the Mexicans close to Puebla, at the Mexican forts of Loreto and Guadalupe. The French army of 6,500–8,000 attacked the poorly equipped Mexican army of 4,000. On May 5, 1862, the Mexicans decisively defeated the French army. The victory represented a significant morale boost to the Mexican army and the Mexican people at large and helped to establish a sense of national unity and patriotism.

The Mexican victory, however, was short-lived. A year later, with 30,000 troops, the French were able to defeat the Mexican army, capture Mexico City, and install Emperor Maximilian I as ruler of Mexico. Maximilian's rule lasted only three years, from 1864 to 1867. By 1865, "with the American Civil War now over, the U.S. began to provide more political and military assistance to Mexico to expel the French." After the American Civil War's end, Napoleon III, facing a persistent Mexican guerilla resistance, the threat of war with Prussia, and "the prospect of a serious scrap with the United States", began withdrawing French troops from Mexico in 1866. The Mexicans recaptured Mexico City, and Maximilian I was apprehended and executed, along with his Mexican generals Miguel Miramón and Tomás Mejía Camacho in Cerro de las Campanas, Querétaro. "On June 5, 1867, Benito Juárez entered Mexico City where he installed a new government organizing his administration."

The Battle of Puebla was significant, both nationally and internationally, for several reasons. First, "This battle was significant in that the 4,000 Mexican soldiers were greatly outnumbered by the well-equipped French army of 8,000 that had not been defeated for almost 50 years." Second, since the overall failed French intervention, some have argued that no country in the Americas has subsequently been invaded by any other military force from Europe. Historian Justo Sierra has suggested in his Political Evolution of the Mexican People that, had Mexico not defeated the French in Puebla on May 5, 1862, France would have gone to the aid of the Confederate States of America in the American Civil War and the United States' destiny could have been different.

On May 9, 1862, President Juárez declared that the anniversary of the Battle of Puebla would be a national holiday regarded as "Battle of Puebla Day" or "Battle of Cinco de Mayo".