As we all know, Texas rebelled against Mexico in the mid 1830's and established itself as a republic, and of course later involved its ally the United States in a war against Mexico from 1846-1848 (OOC: Lets say the US gets the Mexican cession area of the OTL or at least a good chunk of the northern half and Texas just gets all the land it claimed).
Throughout that conflict and the time before, there was much talk about Texas joining the Union as another state. However, it obviously never quite came to fruition.
So what would happen if the Republic of Texas had not remained independent and joined the United States as a state?
OOC: The immediate history from the POD is not too different. The Civil war still happens. Still Lincoln and Davis. Etc.
*I'm not saying the CSA still exist with that, though I guess you could bring up that it indeed does,