A while ago I read something on the Internet asserting that the Mexican victory against Archduke Maximilian and his French army on May 5th at the Battle of Puebla was important in preventing the French from intervening on the side of the Confederacy during the Civil War.
I have been unable to locate the article I originally read, but this story from a Tampa, Florida television station has some information from what looks to be a very interesting history of the holiday, El Cinco de Mayo: An American Tradition, written by David Hayes-Bautista, a professor at UCLA.
From the information in the article from Florida, it is still not clear to me how much of a danger to the US the French represented. But it is clear that the early Mexican-Americans who started celebrating Cinco de Mayo did very much consider the victory at the Battle of Puebla to be a blow against the Confederacy and slavery.
I'll always drink to that. And I'm adding Professor Hayes-Bautista's book to my reading list.