OTL history says that Mexico was settled well before the thirteen mid-Atlantic colonies. It has the natural resources to industrialize, the population to provide that labor force, and the gold to finance it.
Hollywood, on the other hand, says that Mexico is a lawless land of desert and jungle patrolled by banditos and cannibals. Is it no wonder that Mexico is unable to hold off even the most pathetic of foes?
Indeed, it is inevitable that the USA will trounce Mexico in it's first war, or not at all. It will grab up the OTL borders at least, and frequently the entire country. The CSA will declare Manifest Destiny on Mexico and eat the northern third in a war (if they don't buy it). Texas is apparently a fair match, and will slowly destroy their old homeland. Hell, even Adelsverein can defeat this pathetic country.
Typically only happen after adopting an ancient culture. Such as-
The Aztec, or Meshica, culture realies heavily on ritual sacrfice, and is largely ruled by either an Emperor or a savage bloodthirsty Priesthood.
These guys are all about blood, need an early nazi analog, or not willing to make your precious confederacy turn genocidal? Have the Aztecs do it, just as replaceable as regular Mexicans, but more blood thirsty and more organized.
About the only intelligent Mexicans on the planet, they're really into math, and they have actually come up with the date for the end of the world.
On something of a side note they had the first fully developed written language in the Americas.
The end of their civilization is nearly completely unknown, and as a result they make convinient places to change Meso-American history as the author has a large degree of control. Because the Mayan civilization died out long before anyone bothered to look into their writing, no one is exactly sure what they're writing means; as a result TL authors can typically build much of their society however it appeals to them, but most tend to portray them as extremely similar to the Aztecs
May also be very blood thirsty, but tend to be smarter.