The so-called War of the Reform broke out in 1858 soon after liberals drafted a new constitution aimed at reducing the power and influence of the Catholic Church. During the armed conflict, Mexico had two governments: a conservative one in Mexico City led by General Félix Zuloaga and a liberal one in Veracruz led by Benito Juárez, president of the supreme court. The conservatives, who had the support of the pope, won a series of initial skirmishes, but the liberals controlled the ports and were therefore better able to equip their troops. They emerged victorious in January 1861 when they retook Mexico City. The country remained starkly divided, however, with conservatives plotting their revenge.

After the War of Reform, Mexico had virtually no money in its treasury and owed tens of millions of dollars to foreign debtors. The sale of expropriated church property brought very little relief. As a result, newly elected President Juárez suspended payment of all foreign debt for two years, a move that prompted an immediate backlash from Spain, France and Great Britain. With the United States too consumed by the Civil War to enforce the Monroe Doctrine, troops from those three European powers began arriving in Veracruz in late 1861. Spain and Great Britain almost immediately withdrew, but about 6,000 French troops pushed inland toward the capital, backed by Mexico’s vanquished conservative leaders.

In 1862, the French had one of the best armies in the world. Arriving at Puebla on May 4, they were coming off a series of victories in Southeast Asia and Northern Africa and were loaded with firepower, including long-range rifles that put the Mexicans’ creaky muskets to shame. They were so overconfident, in fact, that they didn’t even bother to properly prepare their artillery. On the morning of May 5, the French tried to intimidate the Mexicans with screeching bugle calls and advanced bayonet maneuvers. But after a full day of fighting, including three unsuccessful uphill charges, they were forced to retreat due to heavy casualties.

Mexico’s victory at Puebla slowed, but did not stop, France’s assault. In the wake of the battle, an infuriated Emperor Napoleon III ordered that almost 30,000 more troops be sent to Mexico. This time around, under a new commander, they were able to overrun Puebla and easily conquer Mexico City. Juárez and his supporters then fled to the mountains to conduct guerilla operations while Napoleon III installed Ferdinand Maximilian von Habsburg, second in line to the Austro-Hungarian throne, as Mexico’s ruler.

Back in France, Napoleon III was growing increasingly concerned that Prussia, fresh off victories against Denmark and Austria, would next try to reclaim the perpetually disputed territories of Alsace and Lorraine. Meanwhile, the Civil War had wrapped up, and U.S. officials were exerting diplomatic pressure on the French and supplying weapons to Juárez’s army. With his coffers running low, Napoleon III decided in 1866 to end France’s occupation of Mexico. Maximilian unwisely stayed and briefly fought on, surrendering only after his troops were routed at Querétaro. On June 19, 1867, he and his top generals were executed by a firing squad.

Porfirio Díaz, Mexico’s longest-serving president, was a relatively unknown cavalry commander until the Battle of Puebla, where he outflanked the French on their third charge and sent them into a disorderly retreat. Over the next few years he won battles at Oaxaca and elsewhere. Following the war, however, Díaz became so disillusioned with his onetime friend Juárez that he unsuccessfully ran for president against him. He later launched a couple of coup attempts, finally seizing power in 1876. Except for one four-year break, his reign lasted until 1911, when he was finally deposed by the Mexican Revolution.

Juárez declared Cinco de Mayo a holiday immediately after the Battle of Puebla, but for many Mexicans it has always taken a backseat to such events as the September 16 Independence Day, which commemorates the start of hostilities against Spanish rule in 1810. In the United States, on the other hand, Cinco de Mayo gained traction during the 1960s, when Chicano activists began looking for a way to honor their history and culture. Today, the biggest Cinco de Mayo celebrations are held in U.S. cities with large Hispanic populations, such as Los Angeles, which every year attracts hundreds of thousands of people to its Fiesta Broadway festival.