Tex. Gov. Code
Sec. 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.
Sec. 431.001. DEFINITIONS. In this chapter:
(1) “Reserve militia” means the persons liable to serve, but not serving, in the state military forces.
(1-a) “Servicemember” has the meaning assigned by Section 161.551, Health and Safety Code.
(2) “State militia” means the state military forces and the reserve militia.
(3) “State military forces” means the Texas National Guard, the Texas State Guard, and any other active militia or military force organized under state law.
(4) “Texas National Guard” means the Texas Army National Guard and the Texas Air National Guard.
http://www.statutes.legis.state.tx.us/Docs/GV/htm/GV.431.htm#431.001
Tex. Const. art. 4, § 7
COMMANDER-IN-CHIEF OF MILITARY FORCES; CALLING FORTH MILITIA. He shall be Commander-in-Chief of the military forces of the State, except when they are called into actual service of the United States. He shall have power to call forth the militia to execute the laws of the State, to suppress insurrections, and to repel invasions.