Cinco de Mayo commerates the Battle of Puebla. In the 1860s, the French took advantage of the United States being distracted by the Civil War and invaded México. The French were very pro-Confederate at the time. They planned to use México as a supply line to assist the Confederates. On 5 May, 1863, a rag tag group of Mexican soldiers defeated invading/occupying French troops in an embarrassing defeat. This stopped the French plans to support the Confederacy and fueled a Mexican insurgence to drive out the French and return them to Europe.

In the early 20th century, a group of Latinos in Texas (Mexicans, Guatamalans, Hondurans, etc) wanted to celebrate a “Latino pride” sort of day. Ironically, they chose Cinco de Mayo since it was an unknown, never celebrated holiday that wasn’t country X’s independence day. And it took off from there.