The Army National Guard is the oldest component of the United States armed forces. Militia companies were formed with the first English settlement at Jamestown in 1607. The first militia regiments were organized by the General Court of the Massachusetts Bay Colony in 1636, and from the Pequot War in 1637 until the present day, the Army National Guard has participated in every war or conflict this nation has fought. The militia stood their ground at Lexington Green in 1775 when the opening shots of our War of Independence were fired. They fought the British and their Indian allies from the Great Lakes to New Orleans during the War of 1812, and provided 70% of the troops that fought in the Mexican War.
The majority of the troops that fought in the Philippines during the Spanish American War were National Guardsmen, and the greatest number of combat divisions to fight the Germans during World War I came from the Guard - including six out of the eight that the German General Staff rated as "Excellent" or "Superior."
The Air National Guard was established as a separate reserve component of the U.S. Air Force on 18 September 1947, National Guard aviators have played significant roles in all of America's wars and most of its major contingencies since the First World War era. They have also aided their states in coping with natural disasters and civil unrest since the mid 1920s.
The Guard doubled the size of the Regular Army when it was mobilized in 1940, more than a year before Pearl Harbor, and contributed 19 divisions to that war, as well as numerous other units, to include Guard aviation squadrons. Over 138,000 Guardsmen were mobilized for Korea, followed by numerous smaller mobilizations for the Berlin Crisis, Vietnam, and numerous strikes and riots at home. Over 63,000 Army Guardsmen were called to serve in Desert Storm, and in the decade since then, Guardsmen have seen a greater role than ever before conducting peacekeeping in Somalia, Haiti, Saudi Arabia and Kuwait, Bosnia, and Kosovo.
Excerpted from the National Guard's official web site.