What happened on the fifth of May anyway? The Battle of Puebla, May 5, 1862. At Puebla a French invasion army on its way from Vera Cruz to Mexico was confronted and defeated by Mexican forces. In the end Napoleon III of France sent a new army, which did conquer Mexico and occupy it for a few years. But the victory of the smaller and less professional Mexican forces stiffened Mexican resistance and national pride, and Mexicans in California apparently soon launched the tradition of May 5 celebrations there. More here at About Latin American History. (Cinco de Mayo is observed, apparently, in Puebla State.)
American sites often suggest that the delay in the French conquest of Mexico caused by Puebla prevented Napoleon III from giving material support to the American Confederacy, as he had planned. By the time he had Mexico under control, it was too late to stem the collapse of the American South's resistance. With the re-united United States able to assist the Mexican resistance, the French were run out of Mexico in 1867.
