The American west was taken from Mexico because of the illegal immigration of American settlers.
The Mexican government at first welcomed American settlers into Texas, because they couldn’t get enough Mexicans to settle it. As a result, the few Mexicans there were constantly raided by bands of Comanche, Navajo, Apache and other warrior Indian tribes. Their cattle, sheep and horses were stolen, crops were destroyed, men and women killed or taken prisoner. The Mexicans wanted the Americans to help fight off the Indians.
It worked, all too well, and the government in Mexico realized they had admitted a Trojan horse. The incoming Americans were taking over the territory, and the government in Mexico City ordered it to stop. But it was too late. The Americans kept coming, illegally, and finally in 1835 they rebelled, and the following year the independent Republic of Texas was formed.
When Texas formally joined the Union in 1846 it included present day Texas, the eastern half of New Mexico, the Oklahoma panhandle, southwest Kansas, central Colorado, and a part of southern Wyoming. America profited mightily by the illegal immigration of its citizens into Mexico.
California was won in much the same way — illegal immigration of American settlers. Mountain man Joe Walker led the way, crossing the Sierras in 1833, but the real action began in 1841, with the Bartleson -Bidwell party. Its principal significance is that it included seventeen year old Nancy Kelsey, and her year old daughter Ann.
The story of this party meandering from Missouri to the eastern base of the Sierras, and crossing it on horseback and on foot, is available many places. The best I’ve read is The California Trail: An Epic with Many Heroes. , by George R. Stewart. As they descended from ten thousand foot Sonora Pass, Nicholas Dawson, a young man in the party, later wrote, “I looked back and saw Mrs Kelsey a little way behind me, with her child in her arms, barefooted, and leading her horse — a sight I shall never forget.”
Nancy Kelsey, and a few other American women who came after she did, sewed the first flag of California, the Bear Flag, the one that was flown in the Bear Flag Revolt of 1846. California was taken from Mexico as the spoils of the Mexican-American War. But the war wasn’t really necessary. There were a lot more where Nancy Kelsey came from, and they weren’t going to be stopped. The discovery of gold in 1848 meant that the entire war had been unnecessary. Hundreds of thousands of Americans were on the way now, and California was going to be theirs.
Some Mexican-Americans talk about the Reconquista, or how they are going to overwhelm the Anglos with illegal immigration, and take the west back from the Americans who took it from them. If left unchecked, that’s exactly what they’d do. Americans realize that, and elected Donald Trump to stop it.
Which he is in the process of doing. Elections matter. If you won’t defend your country, it will be taken from you.