"When National Guard units are not under federal control, the governor is the commander-in-chief of the units of his or her respective state or territory (such as Puerto Rico, Guam and the Virgin Islands). The President of the United States commands the District of Columbia National Guard, though this command is routinely delegated to the Commanding General of the DC National Guard. States are free to employ their National Guard forces under state control for state purposes and at state expense as provided in the state's constitution and statutes. In doing so, governors, as commanders-in-chief, can directly access and utilize the Guard's federally assigned aircraft, vehicles and other equipment so long as the federal government is reimbursed for the use of fungible equipment and supplies such as fuel, food stocks, etc. This is the authority under which governors activate and deploy National Guard forces in response to natural disasters. It is also the authority under which governors deploy National Guard forces in response to man-made emergencies such as riots and civil unrest, or terrorist attacks."
Wiki on The National Guard of the United States
"The Act, as modified in 1981, refers to the Armed Forces of the United States. It does not apply to the National Guard under state authority acting in a law enforcement capacity within its home state or in an adjacent state if invited by that state's governor." Wiki on The Posse Comitatus Act
Having been both an enlisted (sergeant E-5) National Guardsman and an officer (colonel O-6) of the Regular Army of the United States, I have a deep interest in the employment of these forces.