The Reform War immediately preceded the French intervention commemorated by Cinco de Mayo.
The Reform War, also called the War of the Reform, was a Mexican civil war fought from 1857 to 1861. Liberal forces supporting President Benito Juárez battled conservatives who opposed liberal constitutional reforms, including measures reducing the Catholic Church’s institutional power.
Mexico’s government emerged from the conflict financially exhausted. In 1861, Juárez suspended payments on foreign debts, prompting Britain, France, and Spain to send forces to Mexico. Britain and Spain eventually withdrew after negotiating with Mexico, but France continued its intervention. French troops advanced toward Puebla, where Mexican forces defeated them on May 5, 1862.
Cinco de Mayo therefore commemorates a battle arising from the political and financial crisis that followed the Reform War. It is not Mexico’s Independence Day, which is celebrated on September 16.