Sounds like you’ve not actually studied the history of the Texas Rangers. THEY WERE NOT FORMED FOR THE BORDER. They were formed to protect settlers from the hostiles. The actual border protection came later. There was some border protection when we were the Republic of Texas...The border protection came after we joined the union. Rangers accompanied federal troops to help protect the border. Since the Rangers were such skilled fighters against the hostiles...the federal troops needed their help. Now the feds would not venture into Mexico when after hostiles...but the rangers sure would.
After we Texans won independance from Mexico, they claimed the new border was the Nueces river.
We disagreed, stormed their capital, and inspired agreement for the border to be the Rio Grande.
The Texas Rangers were formed to patrol that area between the Nueces & Rio Grande, and ensure that inspiration did not fade.
Am I spelling 'Texas' right? If there is anymore you wish to learn, I'll be here all week.