The United States National Guard is a significant part of the USA's military reserve as well as a state-level militia and disaster relief force. The National Guard should not be confused with the Reserves of the various services which serve primarily as training units for replacements to active component forces.
Duties
National Guard units can be mobilized at any time by presidential order to supplement regular armed forces, and upon declaration of a state of emergency by the governor of the state in which they serve. Unlike Army Reserve servicemembers, National Guard members cannot be mobilized individually (except through voluntary transfers), but only as part of their respective units.
History
The National Guard is the development of the American militia during and before the revolutionary struggle against Britain. Following independence, the Constitution empowered Congress to "provide for organizing, arming, and disciplining the militia." But the appointment of officers and training of the militia was given to the states to regulate.
Throughout the 19th century the regular Army was small, and so the militia provided the majority of the troops during the Mexican War, the start of the American Civil War, and the Spanish-American War. In 1903 the militia was renamed the National Guard and organized as a Reserve force for the Army. In World War I, the National Guard made up 40% of the U.S. combat divisions in France. In World War II the National Guard made up nineteen divisions. 140,000 Guardsmen were mobilized for Korea and over 63,000 for Operation Desert Storm. They have also participated in the US peacekeeping forces in Somalia, Haiti, Saudi Arabia, Kuwait, Bosnia, and Kosovo as well as for natural disasters, strikes, riots and security for the Olympics when they have been in the states.
The National Guard while under state command is not subject to thePosse Comitatus Act <
http://www.baltimorechronicle.com/posse_aug02.html> and can engage in law enforcement activities. These law enforcement powers generally disappear when the National Guard is Federalized.
Following WW II, the National Guard aviation units became the Air National Guard. There is no Naval National Guard due to the constitutional provision against states having ships of war in time of peace, though both New York and Maryland have incorporated Naval Militia units.
United States Code
TITLE 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 67 - MILITARY AND NAVY
Section 1385. Use of Army and Air Force as posse comitatus
Whoever, except in cases and under circumstances expressly
authorized by the Constitution or Act of Congress, willfully uses
any part of the Army or the Air Force as a posse comitatus or
otherwise to execute the laws shall be fined under this title or
imprisoned not more than two years, or both.