Texas Government Code - GOV'T § 431.071. Military Duty

(a) The reserve militia is not subject to active military duty, except that the governor may call into service the portion of the reserve militia needed for the period required in case of war, insurrection, invasion or prevention of invasion, suppression of riot, tumult, or breach of peace or to aid civil officers to execute law or serve process.

(b) The governor may assign members of the reserve militia who are called into service to existing organizations of the state military forces or organize them as circumstances require.