Washington (AFP) - The National Guard is a centuries-old militia network that has served crucial roles in several US security operations, testing its mettle in Iraq and Afghanistan as well as on riot-plagued streets back home.
Its Missouri units deployed on Monday to help police in the heartland town of Ferguson, the latest hot spot where the citizen soldiers have been ordered in to help keep the peace and secure public safety.
Whether rushing to help the hurricane-battered US Gulf Coast in 2005, conducting counter-terrorism or peacekeeping operations abroad, or facing off with deadly consequences against unarmed anti-war demonstrators in 1970, the National Guard has served important but often controversial roles.
Its roots pre-date the United States, tracing back to the 17th century regiments of the Massachusetts Bay Colony. Such militias formed the bulk of men doing battle against British forces in the Revolutionary War, or US War of Independence.
The state militias were further empowered under the US Constitution and their members served in large numbers in the Civil War, World War I and other conflicts.
In 1933 Congress brought them under a nationalized system that formed the basis of a US military reserve force.
Today's US Army and Air Force National Guard is 460,000 strong, mostly reservists -- teachers, farmers, engineers and the like who commit to serving "one weekend a month and two weeks a year."
But the reservists have been called up in huge numbers since the terror attacks of September 11, 2001, for tours of duty lasting as long as a year. They have evolved from being the nation's strategic reserve to serving a fuller operational role in battle.
- Missions foreign and domestic -
More than 700,000 have served one or more tours in Afghanistan or Iraq, and 497 National Guard members have been killed in the global war on terror.
"Many National Guardsmen have spent so much time on active duty since 9/11 that they are virtually indistinguishable from the regulars in terms of their military capabilities," Charles Dunlap, director of the Center on Law, Ethics and National Security at Duke University School of Law, told AFP.
Even the National Guard itself stresses its units are designed for war.
"The combat skills and equipment that enable a brigade combat team or flying squadron to mobilize and succeed in Afghanistan also enable them to respond to a natural disaster in the United States," spokesman Rick Breitenfeldt said.
That ability was on show in 2005 when Hurricane Katrina rammed headlong into Louisiana.
More than 50,000 National Guard members from all 50 states deployed to the Gulf Coast in one of the largest domestic response operations in US history.
They conducted search and rescue, cleared roads, delivered relief supplies, and helped local police restore order after the deluge brought chaos to New Orleans.
But decades earlier, the militias had also gained notoriety during landmark civil rights confrontations.
In 1957, the Arkansas governor used members of the state's National Guard to bar black students from entering Little Rock Central High School.
President Dwight Eisenhower reversed the move, ordering the federalization of the Arkansas National Guard and using its members to safeguard the desegregation of the school.
The Guard helped restore order during rioting in the Watts neighborhood of Los Angeles in 1964, in Washington in 1968 after civil rights leader Martin Luther King's assassination, and again in Los Angeles following 1992 race riots.
In 1970 the National Guard suffered its darkest hour, shooting dead four unarmed students at Ohio's Kent State University during protests against the US-led war in Vietnam.
Dunlap said military officials should ensure that reservists play a purely support mission in Ferguson, and not antagonize an already tense community.
"Any time you use the National Guard to quell civil disorder you risk eroding the respect and affection the American people have for the military," potentially undermining future army recruitment efforts, Dunlap said.