American immigrants from Mexico, and their descendants, are one of the fastest-growing segments of the population, according to the latest U.S. census — a fact that comes to mind especially as May 5, Cinco de Mayo, rolls around.

Many Americans think of Cinco de Mayo mainly as the Mexican equivalent of St. Patrick’s Day: an ethnically flavored excuse to drink (and, more importantly, to sell) beer.

And many think that Cinco de Mayo is Mexico’s Independence Day.

But it’s not — in fact, it’s celebrated much more widely in the United States than it is in Mexico because, as a holiday, its origins are here.

“Mexicans are usually surprised by the magnitude of the celebrations of Cinco de Mayo in the United States,” explained Margarita Sanchez, a former journalist from Colombia who is now a Spanish professor at Wagner College. “The date of May 5, 1862, was when Mexican troops won the Battle of Puebla against invading French forces. That victory proved to the Mexican people that it was possible for them to stand up to and defeat foreign invaders.”

The victory at Puebla did not hold; French forces eventually overcame the Mexican army and briefly ruled our southern neighbor — which is why Cinco de Mayo is not celebrated in Mexico with anything like the enthusiasm that accompanies the observance of Mexican Independence Day on Sept. 16.

But when news of the victory at Puebla reached the Mexican miners working the gold strikes in central California in 1862, a little more than a year into the American Civil War, “they spontaneously fired off rifle shots and fireworks, sang patriotic songs and made impromptu speeches,” according to the Southern California Quarterly.

The news from back east about repeated Union defeats was grim, and Californians of all stripes were deeply worried over the future of freedom in the Americas. The French invasion of Mexico had been timed to take advantage of the Civil War, lessening the likelihood of an American intervention against the European invader.

This was what motivated Californians, in 1863, to initiate a celebration of the victory at the Battle of Puebla — to show support for the struggling Mexican government and, by extension, for the cause of freedom throughout the Americas. In California, Cinco de Mayo has been celebrated ever since.

Its celebration has expanded across the U.S. as American immigrants from Mexico and their descendants have settled more and more widely across the country.

“For them, Cinco de Mayo symbolizes freedom, the desire to keep an identity and a soul,” said Professor Sanchez. “Even if many in the United States don’t understand the historical background, it is an opportunity to celebrate the rich contribution made to America by the heritage of Mexican culture in this country.”

Video above: More than 1,000 people took part in the 2011 Cinco de Mayo celebration at Faber Park in Port Richmond, Staten Island, and organizers said it was the first time they tried to include people from outside the Mexican community. (News story broadcast on New York 1, May 8, 2011)