Authorities stand in tear gas while trying to clear protesters at the metropolitan detention center in downtown Los Angeles, on Sunday. Jae Hong/AP

President Donald Trump deployed the National Guard to Los Angeles on Sunday – the first time a US chief executive has used such power since 1992, when the Los Angeles riots erupted after four White police officers were acquitted in the beating of Black motorist Rodney King.

What is the National Guard? It’s part of the US military with a double line of command, answering to both state governors and the president. Its members are part of the primary combat reserve of the Army and the Air Force. Serving on a part-time basis, they can be deployed overseas for combat or support missions, but mostly operate in the US for domestic emergencies like natural disasters.

Every state and territory (plus Washington, D.C.) has a National Guard. Any state governor or the US president can call on the Guard at any moment – setting it apart from other military branches.

Previous state deployments: The National Guard has been mobilized by states for high-profile situations several times in recent years. During the pandemic, Guard members in some states helped to make face masks, field calls from concerned citizens, disinfect facilities and assembling testing kits. Following the January 7, 2021 insurrection at the Capitol, several states sent Guard members to Washington for additional security.

Federal deployments: It’s rare for the president to federally deploy the National Guard the way Trump did, without the request from a governor. Abraham Lincoln had mobilized troops to fight the Confederacy and later support Reconstruction, but National Guard would not evolve into today’s form until the 20th century. After Lincoln, no president deployed the Guard until 1957, when they were sent ensure public order during the desegregation of Central High School in Little Rock, Arkansas.

They were also federally deployed in 1967 to restore public order during the Detroit riots; in 1968 following the assassination of civil rights icon Dr. Martin Luther King Jr.; and in 1970 during the New York postal strike, according to the National Guard.