El cinco de mayo ("Fifth of May" in Spanish) is a national holiday in Mexico. It commemorates the victory of Mexican forces led by General Ignacio Zaragoza over the French expeditionary forces in the Battle of Puebla on May 5, 1862.

Under the pretext of forcing payment for Mexico's outstanding and crippling debt, Britain, Spain and France sent troops to Mexico. The democratically elected government of President Benito Juarez made agreements with the British and the Spanish, who promptly recalled their armies, but the French stayed. Emperor Napolean III wanted to secure French dominance in the former Spanish colony, including installing one of his relatives, Archduke Maximilian of Austria, as ruler of Mexico. Although Abraham Lincoln roundly condemned Napoleon's imperialist ambitions in a neighboring country, he was unable to assist Mexico as the United States was embroiled in the American Civil War at the time.

Confident of a quick victory, 6,500 French soldiers marched on Mexico City to seize the capital before the Mexicans could muster a viable defense. Along their march, the French already encountered stiff resistance before Zaragoza struck out to intercept the invaders.

The battle between the French and Mexican armies occurred on May 5 when Zaragoza's ill-equipped militia of 4,500 men encountered the better armed French force. However, Zaragoza's small and nimble cavalry units were able to prevent French dragoons from taking the field and overwhelming the Mexican infantry. The dragoons removed from the main attack, the Mexicans routed the remaining French soldiers with a combination of their tenacity, inhospitable terrain, and a stampede of cattle set off by local peasants. The invasion was stopped and crushed.

Zaragoza won the battle but lost the war. The French Emperor, upon learning of the failed invasion, immediately dispatched another force, this time numbering 30,000 soldiers. By 1864, they succeeded in defeating the Mexican army and occupying Mexico City. Archduke Maximillian became Emperor of Mexico.

Maximilian's rule was short-lived. Mexican rebels opposed to his rule resisted, seeking the aid of the United States. Once the American Civil War was over, the U.S. military began supplying Mexicans with weapons and ammunition, and by 1867, the rebels finally defeated the French and deposed their puppet Emperor. The Mexican people then reelected Juárez as president.

Contrary to popular belief in the United States, Cinco de Mayo does not mark Mexican independence day. That distinction is reserved for Dieciséis de septiembre ("September 16"), which is celebrated on the night of September 15 through to the early morning hours of September 16 with a re-enactment of the Grito de Dolores – the formal call for an end to Spanish rule in 1810 – at all offices belonging to the executive branch of government, from the president down to the municipal governments.

On 5 May 1901, Ignacio Bravo telegraphed the news of the end of the Caste War of Yucatan with the Mexican victory against the state of Chan Santa Cruz.

The rise in Cinco de Mayo's popularity in the United States can be attributed to the Chicano student movement of the late 1960s. Inspired by student-activists nationwide, members of the MECha organization in California sought to find a day of celebration that highlighted their largely Mexican ancestry. "El Dieciséis de septiembre" (September 16) seemed like an obvious choice; however, this day proved too early in the school-year for college students to effectively organize rallies and celebrations. Thus Cinco de Mayo became the de facto alternative for these student assemblies. Over the years this holiday grew outside university circles and its activist roots, and was absorbed by mainstream culture in the Southwest United States. For many Mexican-American communities Cinco de Mayo is an important way to proudly honor Mexican heritage, overshadowing Mexico's Independence Day in significance. Non-Mexican Americans also participate in the celebrations, much in the same manner that non-Irish Americans observe St. Patrick's Day, with holiday-themed parties marked by the consumption of Mexican food, tequila and Mexican beer.

Although honored today as a national holiday in Mexico, Cinco de Mayo is more of a regional celebration, particularly in the state of Puebla where the famous battle took place.