And then, we joined the United States.
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.