If I remember correctly, after the original colony's declared independence and became the fledgling United States, new states were added by subdividing territorial land after (sometimes long after) such land was acquired through treaty and/or purchase. So the route to statehood was from wilderness to territory to state.
Texas was unique in that it was the only candidate for statehood to have been an independent Republic when it chose to align itself with the nascent United States. They also ask for and got an "escape clause" to allow themselves to withdraw from that union at their discretion.
Regards,
GtG
So the "War of Northern Aggression" caulked up that loop-hole for all time, not only for Texas but for all of the states...
Regards,
GtG
Well, I think the Union’s take on that would be that the CW revoked that. I think the Union goobermint might be in for a surprise.