[MRM]

If I understand correctly, it's a historical accident that Cinco de Mayo ended up being popularized in the US as a Mexican celebration. The battle of Puebla was fought either in Southern California or just south of the border as an extension of the Civil War. It was an important to Mexicans and Americans in the area because it marked the end of French involvement in the area, which could have given them a foothold to support the Confederacy. The locals celebrated the memory of the battle, and the celebration spread with the population as Mexican-Americans moved into other parts of the country.

Eventually the catchy name and handy excuse for an early summer party combined with corporate marketing to make the day a hit in popular culture. I suppose it's not a lot different than St. Patrick's Day, which the Irish celebrate by watching American parades on TV.