James K. Polk was the U.S. president when Texas was annexed with the United States in 1845. He was also very adament about where the border between Texas and Mexico was....the Rio Grande. Polk stood his ground and led the U.S. army against Mexico in the coming years under the direction of General Zachary Taylor. The Texas border was cemented with the outcome of the war. The Rio Grande it would be. I kind of wish it was still the old border from the late 1840's. That way Texas would include Santa Fe, Taos, and my favorite fishing spot- Creede, Colorado. That would be cool.