You might already know that Cinco de Mayo is Spanish for “fifth of May.”
This Mexican holiday celebrates the unlikely victory of 4,000 rather poorly equipped Mexican soldiers against well-armed French forces that were 8,000 men strong!
And this at a time when the French army was considered the best in the world and had not been defeated for almost 50 years!
This victory was won in 1862, near Puebla, at the Mexican forts of Loreto and Guadalupe.
It is perhaps surprising that this holiday is little celebrated in Mexico outside the state of Puebla. It is a much bigger deal in the United States, where people of Mexican and Latino heritage are often joined by non-Hispanic people in celebrating Mexican heritage and pride. It's a time when many school teachers and children's librarians teach about Mexican culture, communities hold festivals with dancing and food, and even the U.S. Congress issued a resolution to call upon the people of the U.S. to observe the day with appropriate activities (in 2005).
According to Wikipedia, since the Battle of Puebla, no country in the Americas (North or South) has been invaded by a European military force.
That's not to say that the Mexican victory in the Battle of Puebla repelled the French army. Thirty thousand troops and an entire year later, the French were able to defeat the Mexican army, capture Mexico City, and install Emperor Maximilian I as ruler of Mexico. It took the Mexican people another three years to re-capture their capital, execute Maximilian, and win back their country.