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.
By 1823, there were serious problems with raids by the Comanche, Tonkawa and Karankawa Indians. Under Mexican law, Austin was authorized to form a militia to ward off Indian raids, capture criminals and patrol against intruders. In May, while Austin was in Mexico City, his lieutenant, Moses Morrison, used this authority to assemble a company of men to protect the Texas coast from the Tonkawa and Karankawa Indians.
After returning to Texas in August of 1823, Austin asked for additional ten men to supplement Morrison's company. He called for "ten men...to act as rangers for the common defense...The wages I will give said ten men is fifteen dollars a month payable in property." These two companies are regarded as the first ancestors of the modern Texas Rangers.
I just read 2 books on the Texas Rangers. However, there's a brief history here. http://www.texasranger.org/