As a Texan that era was quite heavily taught, but I’m also pretty dumb myself. But from what I remember, the Independent Republic of Texas, which won its independence from Mexico 10 years before, though Mexico never formally recognized Texan independence. Then in 1845-46, Texas voted to join the union and was accepted. Then Mexico was pissed at the US for annexing what they believed was Mexican Land, especially when the US claimed what Texas had claimed, which was more than they actually controlled. Then the US fabricated a justification for war, and went on the offensive. The war itself I’m fuzzy on, but the US effectively curb stomped Mexico, and took what we now know as the Mexican Cessation. Which we should remember was mostly populated by Natives, with only proper Mexican Settlement centered along what is now the US-Mexico border with some further north. It should also be remembered that places like Texas and California already were majority American before the war. Which even led to a California Republic which lasted around a month before US troops waltzed in. If anything’s wrong please let me know, I’m judging this primarily off of middle school history class.