( CNN ) -- 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 is n'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 . WHEN : 2005 . WHY : In addition to assisting with rescues , National Guard members were called in to help support local law enforcement . Lt. Gen. Russel HonorÃ© , 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 . HonorÃ© 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 . WHEN : 1992 . 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 . WHEN : 1970 . 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 . WHEN : 1957 . 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 . WHEN : 1794 . 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 did n'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 . Why bring National Guard to Ferguson ? Read more about the flash point in the Heartland at CNN.com/US . CNN 's Mary Rose Fox and Jamie Maglietta contributed to this report .
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 are also used to help with natural disasters like hurricanes and tornadoes.