On March 20, 1965, President Lyndon B. Johnson notifies Alabama’s Governor George Wallace that he will use federal authority to call up the Alabama National Guard in order to supervise a planned civil rights march from Selma to Montgomery.

Intimidation and discrimination had earlier prevented Selma’s black population–over half the city–from registering and voting. On Sunday, March 7, 1965, a group of 600 demonstrators marched on the capital city of Montgomery to protest this disenfranchisement and the earlier killing of a black man, Jimmie Lee Jackson, by a state trooper.

In brutal scenes that were later broadcast on television, state and local police attacked the marchers with billy clubs and tear gas. TV viewers far and wide were outraged by the images, and a protest march was organized just two days after “Bloody Sunday” by Martin Luther King, Jr., head of the Southern Christian Leadership Conference (SCLC). King turned the marchers around, however, rather than carry out the march without federal judicial approval.

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After an Alabama federal judge ruled on March 18 that a third march could go ahead, President Johnson and his advisers worked quickly to find a way to ensure the safety of King and his demonstrators on their way from Selma to Montgomery. The most powerful obstacle in their way was Governor Wallace, an outspoken segregationist who was reluctant to spend any state funds on protecting the demonstrators. Hours after promising Johnson—in telephone calls recorded by the White House—that he would call out the Alabama National Guard to maintain order, Wallace went on television and demanded that Johnson send in federal troops instead.

Furious, Johnson told Attorney General Nicholas Katzenbach to write a press release stating that because Wallace refused to use the 10,000 available guardsmen to preserve order in his state, Johnson himself was calling the guard up and giving them all necessary support. Several days later, 50,000 marchers followed King some 54 miles, under the watchful eyes of state and federal troops.

Arriving safely in Montgomery on March 25, they watched King deliver his famous “How Long, Not Long” speech from the steps of the Capitol building. The clash between Johnson and Wallace—and Johnson’s decisive action—was an important turning point in the civil rights movement. Within five months, Congress had passed the Voting Rights Act, which Johnson proudly signed into law on August 6, 1965.