Alexander Meigs Haig, Jr., was born in Bala-Cynwyd, a suburb of Philadelphia, Pennsylvania. He graduated from the United States Military Academy at West Point in 1947, launching a distinguished military career in which he eventually achieved the rank of four-star general. Haig graduated from the Naval War College in 1960, earned a master's degree from Georgetown University in 1961, and attended the Army War College in 1966. In late 1968, Henry Kissinger, who was then reorganizing the foreign affairs staff for President-elect Nixon, appointed Haig as his military adviser on the National Security Council. In 1970 Haig was named deputy assistant to the president for national security affairs, and his diplomatic authority expanded to include secret peace talks in Paris aimed at ending the Vietnam War (1959-1975).

When Nixon's top two aides—White House Chief of Staff H. R. Haldeman and domestic policy adviser John Ehrlichman—resigned in 1973 amid growing evidence of their involvement in the Watergate cover-up, Haig reluctantly left the military to become Nixon's chief of staff. Haig reportedly played a significant role in talking the president into resigning, which Nixon did on August 9, 1974. Six weeks later, Haig resumed his military career as supreme allied commander of North Atlantic Treaty Organization (NATO) forces in Europe.

Haig was appointed secretary of state in 1981 during President Reagan's first term in office, but resigned in 1982 amid disagreements with other administration officials. He wrote one book about the Reagan administration, Caveat: Realism, Reagan, and Foreign Policy (1984), and ran an unsuccessful bid for the 1988 Republican nomination for president. In 1992 Haig published his memoirs, Inner Circles: How America Changed the World.