He can order the TX Hwy patrol and TX Rangers (the detectives not the baseball team) to post units near the borders)... Since those are civilian agencies, it is therefore out of the right of the President to order them to stand down.. Also, since a county Sheriff is the highest authority law enforcement officer within his jurisdiction, order his deputies to the border or establish special border deputies for each county and arm them to the teeth... Dare the POTUS, Congress, or the Mexican army to cross them.
There are only 140 Rangers in the entire state, and it's a big state.