The current Mexican flag was adopted on September 16, 1968. Mexico gained its independence from Spain on September 21, 1821, (that' why there's a lot of Mexicans that are descendents from Spain), and at that time they introduced a basic flag of green, white and red stripes (going up and down). The Mexican coat of arms - which represents the 1325 founding of the Aztec capital Tenochtitlan (occupied by Mexico's modern capital, Mexico City) was added in 1823. The coat of arms has changed a lot of times since then and this has resulted in several arrangements to the Mexican flag. In Mexico the flag is called la Bandera Nacional.
 

• White - peace and honesty

• Red - hardiness, bravery, & strength

• Green - hope, joy and love and in many cultures it has a sacred significance

• Mexico's Coat-of-Arms means an ancient Aztec legend is a tale of God telling a band of wandering Aztec Indians that when they came across an eagle eating a snake, on top of a nopal cactus growing out of a rock filled of water.  That's where they were to create their home. This spot eventually became Mexico City. This image with the eagle eating a snake is depicted on the Mexican Coat-of-Arms.