Since Texas was an independent nation before joining the United States, when did the United States of America get any title at all to a square inch of Texas? Did they purchase it?
Yes. It was called the Louisiana Purchase(1803).
The Red River was the border between the US and New Spain, which would become Mexico(1821), then Texas(1845).
The US claimed the entire Red River bed and Spain, Mexico, and Texas never disputed. The river changes course and the federally owned river bed moves.
This was all settled by SCOTUS back in the 1920s.
In that case OK sued Texas because there were slant oil wells in Texas producing oil from in OK. The US govt entered the lawsuit to assert their ownership of the river bed.
Excellent question and I hope someone provides an honest answer!