On April 4, 1841, President William Henry Harrison succumbed to pneumonia one month after his inauguration, becoming the first U.S. chief executive to die in office; Harrison’s vice president, John Tyler, was sworn in as president two days later.

In 1949, 12 nations, including the United States, signed the North Atlantic Treaty in Washington, D.C., establishing NATO.

In 1968, civil rights leader Martin Luther King Jr., 39, was shot and killed while standing on a balcony of the Lorraine Motel in Memphis, Tennessee. King’s death triggered a wave of unrest in cities across the United States that killed 43 people and injured more than 3,000.

In 1973, the twin towers of New York’s World Trade Center were officially dedicated.

In 1975, Bill Gates and Paul Allen founded Microsoft in Albuquerque, New Mexico.