Tex. Gov't Code § 431.112

If military aid is immediately and urgently necessary to prevent or suppress violence under Section 431.111(c) and it is impracticable to secure the aid in time by order of the governor, the district judge of the judicial district, the sheriff of the county, or the mayor of the municipality in which the disturbance occurs may call for aid on the commanding officer of the state military forces stationed in the judicial district, county, or municipality or an adjacent judicial district, county, or municipality. The officer must make the call in writing and shall immediately notify the governor of the action.