Many Americans (gringos) incorrectly assume that Cinco De Mayo is Mexican Independence day. They are misinformed on the significance of May 5th to Mexicans and how the triumphant battle in Puebla, Mexico, which the celebration honors, helped assure American unity and victory for Union forces.
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Long story short, the French hated that the American were becoming a stronger national power. The Mexicans had defaulted on payment as the county went bankrupt. The newly elected President Benito Juarez made deals to pay the loans, the Spanish and English were satisfied, but the French had other ideas. The French wanted to take over Mexico and claim it as part of their empire. Peasant farmers in Mexico fought off the uber-organized French military, the most powerful army in the world. Before the French could take over Mexico and help the Southern Confederates create a divided America, a smaller threat to French interest, the Mexicans kicked thier Parisans booty and won the war.
Happy Cinco De Mayo!!