Cinco de Mayo is celebrated to commemorate the Mexican army winning the Battle of Puebla during the Franco-Mexican War in 1862. Mexican-Americans in California began celebrating Cinco de Mayo the following year to show solidarity with Mexico against French rule. In the United States, the holiday has developed over time into a general celebration of Mexican culture and heritage.
The battle of Puebla was significant because the Mexican army was outnumbered two to one by the superior French army. The battle lasted all day on May 5, 1862 and left nearly 500 French soldiers dead, while Mexico lost fewer than 100 soldiers. While it was not the biggest victory during the war, it was considered a major symbolic victory for the Mexican resistance.
During this time, the United States was engaged in the Civil War, and California sided with the Union. When Mexican-Americans in California heard about the Battle of Puebla victory, they spread the news, which helped give inspiration to the Union forces in the Civil War. When the Civil War ended, the United States was able to help Mexico defeat France and restore independence.
Over time, Cinco de Mayo turned into an American celebration of Mexican culture that includes parades and traditional Mexican music, dancing and food. However, it is not widely celebrated in Mexico, except in the state of Puebla.