The unorganized militia may be called for active duty in case of war, rebellion, insurrection, invasion, tumult, riot, breach of the peace, public calamity or catastrophe, or other emergency, or imminent danger thereof, or may be called forth for service under the Constitution and laws of the United States. Whenever it is necessary to call out any portion of the unorganized militia, the Governor may call for and accept as many volunteers as are required for such service, under regulations provided by this division.