On May 5, 1862, a vastly outnumbered force of about 2,000 Mexican locals repelled and defeated 6,000 well-trained French troops in what became known as the Battle of Puebla.

The victory is celebrated as Cinco de Mayo in Puebla, a city southeast of Mexico City. It’s also recognized in Veracruz, the port on the Gulf of Mexico where the French landed to invade and left when defeated.

The against-all-odds victory never spawned a national holiday in Mexico, however.

In fact, it is recognized more in the United States, where Mexican miners in the Gold Rush town of Columbia celebrated it in 1863, a year after the battle and 13 years after California became a state. Today, Los Angeles, in fact, boasts the world’s largest Cinco de Mayo celebration.

That stated, few folks know why they celebrate Cinco de Mayo and routinely confuse it with Mexican Independence Day (September 16, 1810).

Consider this: Mexico’s victory in the Battle of Puebla ultimately did as much for the United States and preserving the Union as it did for Mexico.

In 1860, still hurting financially from the Mexican-American war of 1846-47 and internal conflicts, Mexico’s bankrupt government announced it would suspend repayment of debts to European countries including Spain, Great Britain and France.

French Emperor Napoleon III, nephew of Napoleon Bonaparte, saw the opportunity not only to get his money back, but also to establish a French colony in North America. Once the American Civil War began in 1861, Napoleon III thought the Union would be too preoccupied with the South to stop him. He also saw it as a chance to trade guns for cotton with the Confederates, whose supply lines were being stymied by Union blockades.

What Napoleon III didn’t bank on was having his professional soldiers whipped in Mexico by a force of volunteers quickly assembled by Mexican General Ignacio Zaragoza. The French returned and captured Puebla in 1863. But many historians credit the Mexicans’ Cinco de Mayo victory a year early with helping the Union prevail. It bought the North enough time to win some major battles against Confederate forces that could not get the weaponry they needed from France.