Again, thanks. I'm interested in whether the Mexican-American War (or, from the Mexican POV, the Invasion of Mexico), had any relation to the Civil War. I see it as basically robbing Mexico at gunpoint, despite a purchase price negotiated for Texas, California etc. It fascinates me that these conflicts over labor, agriculture and industry still exist, to some extent. I see it played out in immigration arguments and so forth. When I read books of war history, I tend to get bogged down in the incremental back-and-forth escalation. My instinct is to summarize and maybe oversimplify.
Two very distinct and very different wars. Since we're not discussing the Mexican-American, I'll only address the Civil War.
Although a scant few years after, the blink of an eye in terms of world history, it was a very different Texas in 1860. In the 12/13 years after the Mex/Amer war, the population tripled. Of the total 600,000 head count, 1/3 of those were slaves. The "Tejanos" were not really an issue in the Civil War. It was the cotton growers (and that was the big money in the state) in the east of the state who were the heavyweight supporters of the Confederacy. Most had emigrated from the "old south" in the past decade or two. Cattle wasn't king yet; it wasn't even the crown prince. Texas' contribution to the war effort was soldiers and horses.... and, the route to the Mexican "gray market".
The history of Texas in the Civil War is checkered, to say the least. The opening and closing months of the war were largely ignored in history books in Texas and Oklahoma when I was a kid. In the beginning, the supporters of the Confederacy, who were not as dominant as the voting results would say (what with armed thugs running the conventions and meetings) respected the large number of Texans who had Union sympathies. Well, if "respected" means "hunted down and killed". This wasn't in the "heat of battle", but the opening weeks of the war. Rather than "get out!", it was "get out if you can; we're gonna give you a head start".
And the end of the war was anarchy. In spite of the government of Texas declaring that they would continue the war without Virginia and South Carolina and Georgia and Mississippi and Alabama, when word got to the troops of Lee's surrender, they revolted and raided everything that had something worth stealing. The investigation afterwards, by the good old boys of the Confederacy who were still in power just without uniforms, determined that it was not the soldiers but the civilian population that did the rioting, looting, ambushing of supply wagons and robbing of banks. This interesting conclusion is in spite of the comments of the General of the West of the Mississippi (for the Confederates) who when he got back from the front and tried to reinstate order, found that he had no troops to command as they'd all lit out. In fact, he's on record as blaming the deserters and in a wonderful bit of self-serving irony, gave them most hell for not being there for him to command. (It was sort of like the Napoleon character in Age of Empires.... "What??!! 'Ow can I be a zheneral with no troops to command?")