Four U.S. presidents have been murdered while in office—all brought down by gunfire. And each of these presidential assassinations helped usher in a wave of important reforms and a new political era.

President Abraham Lincoln, America’s Civil War leader, was assassinated just five days after Confederate General Robert E. Lee surrendered his army at Appomattox Court House, ending the four-year War Between the States.

On the evening of April 14, 1865, Lincoln was attending a performance of Our American Cousin at Ford’s Theatre in Washington, D.C. John Wilkes Booth – a 26-year old actor, Confederate sympathizer, and white supremacist – slipped into the Presidential Box and shot Lincoln in the head.

Booth and his conspirators had initially planned to kidnap Lincoln to save the Confederate States. But as the Confederacy faltered, Booth’s thoughts turned to murder. Booth may have decided to act on his hatred after Lincoln endorsed giving the right to vote to African-American men who had served in the Union Army.

The assassination of President Lincoln was just one part of a larger plot to decapitate the federal government of the U.S. after the Civil War.

Lincoln never lived to enact this policy. He died the following morning on April 15, 1865. His successor Andrew Johnson assumed office and presided over Reconstruction.

Johnson, a Congressman and former slaveholder from Tennessee – and the only Southern senator to remain loyal to the Union during the Civil War – favored lenient measures in readmitting Southern states to the Union during the Reconstruction era.

A proponent of states’ rights, Johnson granted amnesty to most former Confederates and allowed Southern states to elect new governments. As a result, new state governments formed across the South and enacted “black codes.”

These restrictive measures were designed to repress the recently freed slave population. Soon, many African Americans had little choice but to continue working on Southern plantations.

On July 2, 1881, President James A. Garfield was shot twice – in the arm and the back – as he entered the old Baltimore and Potomac Railroad Station in Washington, D.C. The former Ohio congressman, who was en route to Massachusetts to give an Independence Day speech, had been president for just four months at the time.

Garfield survived the initial injury but died two months later from a severe infection. The infection likely was the result of unsanitary surgical practices as doctors attempted to remove the bullet from his back.

Charles Guiteau, the man who shot Garfield, was an unsuccessful lawyer and preacher had stalked the president around Washington, D.C., for weeks before the attack. Months earlier he had written a speech on Garfield’s behalf. The speech was largely ignored, but Guiteau had formed the delusion that it had been a deciding factor in the president’s victory.

Seeking patronage for the “vital assistance,” he had provided the president, Guiteau insisted he should be awarded an ambassadorship in Paris. He turned to revenge when he was denied the post.

The assassination of Garfield by Guiteau, the disgruntled public officer seeker, became the impetus for the Pendleton Civil Service Act. Garfield’s successor, Chester A. Arthur, signed the 1883 act, which reformed the civil service system and established the principle that federal jobs should be awarded based on merit rather than political patronage.

Anarchist Leon Czolgosz shot President William McKinley on September 6, 1901. McKinley was shaking hands at a public reception at the Pan-American Exposition, a fair in Buffalo, New York.

Czolgosz concealed his weapon with a handkerchief and shot McKinley twice in the stomach at close range. McKinley died of his wounds eight days later, on September 14.

McKinley’s assassination led to the creation of the modern Secret Service. Before McKinley’s death, presidential security was lax and often piecemeal. After his death, the Secret Service – originally a branch of the Treasury Department created to investigate counterfeit currency – became the President’s dedicated, permanent security detail.

McKinley’s successor, Theodore Roosevelt, was widely regarded as the first modern president. Roosevelt increased the power of the executive branch by enforcing strong anti-trust legislation at home, while simultaneously increasing the influence of the United States in global politics abroad.

President John F. Kennedy’s assassination took place on November 22, 1963. He was traveling with his wife Jacqueline and the Texas governor John Connally in an open-top convertible during a 10-mile motorcade through Dallas, Texas when gunman Lee Harvey Oswald fired three shots from the sixth-floor window of a nearby building.

Two shots hit President Kennedy in the head, killing him. One bullet struck Connally, who survived the assassination attempt. Oswald was shot and killed two days later by Dallas nightclub owner Jack Ruby as he was being escorted out of police headquarters.

Before his death, Kennedy had proposed civil rights legislation that would outlaw discrimination based on race, religion, sex or national origin. The legislation was controversial and had stalled in Congress before Kennedy’s death. Kennedy’s assassination changed the political dynamics of the civil rights movement.

His successor, vice president Lyndon B. Johnson (who was three cars behind Kennedy in the motorcade), took advantage of the national outpouring of sympathy after Kennedy’s death to push through the historic Civil Rights Act and Voting Rights Act, which helped end racial segregation and provided greater protections to black voters.