The day is in celebration of Mexican army’s victory over French troops in Puebla, MX (about an hour east of Mexico City). The war took place in 1862.
President Benito Juarez had stopped making loan payment to France, Spain, and the UK. The countries decided to invade Mexico and pressure Mexico to pay their debt.
Spain and the UK, however, withdrew their forces when they realized France wanted to conquer and occupy Mexico.
After a couple of defeats, Mexico finally won in the Battle of Puebla.
French forces eventually went on to win many battles and capture Mexico City. In 1866, France withdrew their forces from Mexico.
Ironically, today is not a federal holiday in Mexico.
Sources: wikipedia; http://photographywritingandmore.blogspot.com/2007/05/cinco-de-mayo.html
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