Capt. Jack Hays was a Texas Ranger, Indian Fighter, led this Regiment in the Mexican War, and became the first Sheriff of San Francisco.
The Regiment served with General Winfield Scott on his march from Vera Cruz to Mexico City.

From the official Texas Ranger site:  http://www.texasranger.org/dispatch/1/Hays.htm

Another good site:  http://members.tripod.com/~jack0204/gen/Heskew/jack_c_hays.htm

A little on the war, itself: http://www.texasmilitaryforcesmuseum.org/tnghist9.htm

And, something about the war itself:  http://www.dmwv.org/mexwar/mexwar1.htm

and, a pretty good site:  http://www.sonofthesouth.net/mexican-war/war.htm