Actually, on my current game - USA 1860 - Texas never applied for admission to the USA. I don't know why. I was in an early war against Mexico, as was Texas of course, but by December of 1845 we were both at peace - but they never applied. I don't know why. I know they can decide against it, but I don't remember that happening. So now, in 1860, the Republic of Texas remains, though with only Austin, Dallas and Lubbock. They lost both Amarillo and Houston to Mexico, and I have since conquered them.
Following two wars against Mexico, I now have basically all of the modern USA. Bar of course, some of Texas, and one state which I can't claim as I have claims on one of its provinces, and Mexico has claims on the other two. Oh, and Hawaii and Alaska, of course. I had a huge army for the second war. No contest. As soon as this peace treaty expires, I'm going to start another Mexican war.
Finally, it's 1860, civil war is on the horizon. Or is it? I'm quite certain that if the Republicans don't win the election, then there is no civil war. That may sound like a good thing, but apparently that means slavery is here to stay. The Democrats won the 1859 election but the Republicans (newly formed) were gaining ground on them, so I called another one. Unfortunately, with only a few months to the election, the Democrats lead with 52% to the Republican's 28%.
See, it's a great game.
