Texas Government Code - GOV'T § 431.113. Duty of Commanding Officer

(a) On receipt of a call under this subchapter, a commanding officer immediately shall order the called forces under the officer's command to parade at the time and place appointed and shall notify the governor of the action.

(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 or tumult or the preservation of public peace.  The instructions must be in writing, except that if written instructions are impracticable the instructions must be given verbally in the presence of two or more credible witnesses.  The commanding officer is solely responsible for determining the kind and extent of force to be used and the method of implementing the instructions.