According to my daily planner, today is Día de la Bandera, that is, flag day in Mexico. Wikipedia states that “The date was selected [in 1937] because more than a century earlier (February 24, 1821), the “Plan of Iguala” or “Plan of the three guarantees” was proclaimed by Agustin de Iturbide and General Vicente Guerrero. This plan was based on three principles: Religion, Independence and Unity, which were represented by the flag’s colors.”

Also from Wikipedia, a photograph of a collection of Mexican flags on display at the Mexican History Museum of Monterrey, Nuevo León:

The central device on the flag is the Escudo Nacional, that is, the national shield (even if it isn’t a shield as such). Here is the current standard depiction:

That is, it shows a Mexican golden eagle perched on a prickly pear cactus devouring a rattlesnake, an image that has resonance in both Aztec and European culture.