The Flag of Mexico is a vertical tricolor of green, white, and red with the national coat of arms charged in the center of the white stripe. The current flag was adopted in 1968, but the overall design has been used since 1821, when the First National Flag was created.
Red, white, and green are the colors of the national liberation army in Mexico. The central emblem is the Aztec pictogram for Tenochtitlan (now Mexico City), the center of their empire. Throughout history, the flag has changed four times, as the design of the coat of arms and the length-width ratios of the flag have been modified. However, the coat of arms has had the same features throughout: an eagle, holding a serpent in its talon, is perched on top of a prickly pear cactus; the cactus is situated on a rock that rises above a lake. (source) Thank you Victoria!