- National Guard troops can be activated by governors or the president
- Every state and Washington, D.C., have their own National Guard
- Guard members have helped quell unrest in Los Angeles and New Orleans
- They have also been mired in controversy, such as with the Kent State shootings
They've served in wars in what is now the U.S. since 1637. When natural disasters strike, they're often among the first to head to the epicenter.
And now, National Guard members are trying to keep the peace in Ferguson, Missouri, as the city grapples with the shooting death of an unarmed teenager.
Ferguson certainly isn't the first time the troops have been called up to quell civil unrest.
According to the U.S. Constitution, the militia can be deployed "to execute the Laws of the Union, suppress Insurrections and repel Invasions." And both the president and governors can call up the troops.
Here's a look at other times National Guard members have stepped in during civil turmoil:
WHAT: Hurricane Katrina
WHY: In addition to assisting with rescues, National Guard members were called in to help support local law enforcement.
Lt. Gen. Russel Honoré, who led Task Force Katrina, said National Guard troops "were positioned on every block to establish a sense of safety and source of help for the people in need."
The storm that led to 1,833 deaths also spawned tumult, particularly in New Orleans.
Officers told CNN at the time they lacked manpower and steady communications to properly do their jobs. They said they needed help to prevent the spread of looting and violence that were prevalent in the city.
Honoré said about 50,000 National Guard members responded to Katrina. They "did not leave communities till people were safe and sound," he wrote.
WHAT: Los Angeles riots
WHY: When four police officers were acquitted after the beating of Rodney King, a series of riots over five days left more than 50 people dead and a city wracked with racial tensions.
Like in Ferguson, the Los Angeles conflict started with the controversial treatment of a black man by white police. After King led officers on a high-speed chase, he was struck more than 50 times with police batons and suffered 11 fractures.
More than 9,800 California National Guard troops were dispatched to help restore order. Unlike with Ferguson, where Missouri National Guard troops were summoned by the governor, President George H.W. Bush called the Guard into federal service during the Los Angeles riots.
The LA riots marked the most recent time the National Guard was federalized, National Guard spokesman Jeremy Webster said.
WHAT: Kent State University rally
WHY: About 100 Ohio National Guardsmen were called to Kent State in Ohio to disperse an angry crowd of students protesting the Vietnam War.
Guard members fired tear gas, and some students said they were surprised the guardsmen followed them as they ran away.
After several standoffs, 28 Ohio guardsmen fired into the crowd for 13 seconds, wounding nine students and killing four.
The shootings led to a national protest involving more than 4 million students.
WHAT: Little Rock high school desegregation
WHY: In perhaps the most controversial state deployment of National Guard members, Arkansas Gov. Orval Faubus called on troops to block nine black students from attending Little Rock Central High School.
"That's when I knew that they were just not going to let me go to school ... that they were not there to protect me, too, like the other students," recalled Elizabeth Eckford, one of the "Little Rock Nine." She was 15 at the time.
The nine black students were taunted and spat upon by a white mob when they attended school.
President Dwight Eisenhower eventually sent more than 1,010 federal troops to Little Rock to ensure compliance with court-ordered integration.
WHAT: Whiskey Rebellion
WHY: A popular uprising broke out against a federal excise tax on liquor and the stills that produced it.
After Pennsylvania's governor said he didn't have enough militia to enforce compliance, Secretary of War Henry Knox called for more than 12,000 troops from Virginia, Maryland, New Jersey and Pennsylvania.
But it took two months to get the troops to western Pennsylvania. By the time they reached Pittsburgh, the uprising had been pacified.