The War of U.S. Invasion

Here's a slightly less biased explanation of the conflict between U.S. & Mexico during the 1830s & 40s:
Mexico was still a territory of Spain until about 1820. During 1800-1820, white Europeans settled Texas under a treaty with the royalty of Spain, pledging to become Spanish citizens, learn Spanish, and become Catholic. They didn't.

In the 1830s, Austin encouraged Texas to become independent (not part of the U.S.) and the rebels who followed him fought the battle of the Alamo after much attempted negotiation on the part of the Mexican government. About 200 Texans died and about 400 Mexicans died. This battle angered Texans, and Santa Anna, the leading military general and working president of Mexico, knew he could win no more battles, so he signed a secret peace treaty with Texas. Texas was independent for almost a decade.

Then, the Mayans in the Yucatan demanded independence from Mexico (remember, Mexico is a relatively new government and is still determining it's borders at this time) and Mexico sent its troops to that area. Seeing a weakness in Mexico, U.S. President Tyler asked congress to make Texas a state, and they did - sort of. But they were claiming the western border of Texas to be much farther west than the Mexicans had ever agreed to.
Under the then-current U.S. belief of Manifest Destiny, U.S. President Polk was willing to fight Mexico for Texas (and buy the rest of the land from there to the Pacific), knowing full well that Mexico was currently under poor leadership and their budget was being stretched fighting in the Yucatan. He figured that the U.S. could win a couple of battles in the Texas area, while the Mexican troops were in the Yucatan, and that dangling millions of dollars in their face would force a Mexican surrender.
He was basically right.
Pres. Polk got congress to declare War on Mexico after a skirmish that killed only 16 soldiers. Mexico needed a few weeks to figure out their war strategy, and in that time the U.S. was more than ready to beat them, and they did. For some reason, Santa Anna retreated after a battle in Monterrey. General Scott's march from Yucatan to Mexico City roughly paralleled General Sherman's march to the sea in the U.S. civil war. He massacred Veracruz, marched to Mexico City, massacred them (killing more than twice as many civilians as soldiers) and went straight to Chapultepec Castle. This castle has formerly been the residence of the Emperor of Mexico, but since it's recent independence, was now housing a military academy of young soldiers. Those soldiers chose death rather than surrender (typical for 17 year old boys) and hurled themselves into the front lines of the U.S. soldiers. They were quickly epitomized as the "child heroes" and came to represent a new national fear/distrust of North Americans.

The U.S. invading a young country like Mexico would have been roughly equivalent to Spain invading a young America in 1806. It takes about 2 decades to work out the kinks of a new constitution, build an army, and create national communication and solidarity. Mexico hadn't quite finished that process when the U.S. invaded.
In our defense, although we received almost half of Mexico's land, that area contained no known natural resources and less than 2% of Mexico's citizens. It didn't seem very valuable to the Mexican people. Giving it up to the Americans hurt their pride more than their wallets.

Until 1849 - just 1 year later - when gold was discovered in California. Soon after, the new immigrants to California also discovered the amazing agricultural resources of the area. Today, California is our largest state, and a huge source of American-grown food. If California had been under Mexican control when gold was discovered, the history of both of our countries would have been significantly different.