UCLA Professor David Hayes-Bautista concludes in a new book, "El Cinco de Mayo: An American Tradition," that Latinos living in California and Oregon first marked Cinco de Mayo in 1862 when Mexican forces defeated French invaders in the Battle of Puebla at the same time the Union and Confederacy were slugging it out in the American Civil War.
The two events might not appear at first glance to be related, but Hayes-Bautista said Mexicans living north of the border saw the Civil War and the French incursion into Mexico as part of the same conflict.
"In Mexico today, Cinco de Mayo means the Mexican army defeated the French army," Hayes-Bautista told CNN. "In California and Oregon, the news was interpreted as finally that the army of freedom and democracy won a big one against the army of slavery and elitism."
Hayes-Bautista said French ruler Napoleon III was sympathetic to the slave-owning Confederacy and no friend of democracy in general. Had France overrun the fledgling Mexican democracy, Napoleon III would have established a monarchy that could have caused trouble for the Union on the lightly defended West Coast.
For that reason, Mexicans north of the border and their American neighbors both embraced Cinco de Mayo as a rallying point. For at least a few years after the wars, Mexican and U.S. veterans would appear together at May 5 celebrations.
"From 1862 to 1867, the public memory of Cinco de Mayo was forged in the American West," Hayes-Bautista said.
While memories of Puebla faded quickly, the United States continued to have a strong Mexican presence. The U.S. Census Bureau said its most recent head count has 31.8 million people of Mexican descent living in the United States, which is about 10 percent of the total population.