Texas Government Code - GOV'T § 433.005. Calling of State Military Forces

(a) The chief executive officer or governing body of a county or municipality may request the governor to provide state military forces to aid in controlling conditions in the county or municipality that the officer or governing body believes cannot be controlled by the local law enforcement agencies alone. On receiving the request, the governor may order a commander of a unit of the state military forces to appear at a time and place the governor directs to aid the civil authorities.

(b) After the forces have appeared at the appointed place, the commanding officer shall obey and execute the general instructions of the civil authorities charged by law with the suppression of riot, the preservation of public peace, and the protection of life and property. The instructions must be in writing, except that if written instructions are impracticable, the instructions may be given verbally in the presence of two or more credible witnesses. The commanding officer shall exercise his discretion as to the proper method of practically accomplishing the instructions.