Alexander M. Haig's life threaded through some of the most tumultuous episodes of the second half of the 20th century. An Army officer in Vietnam, a presidential advisor during the Watergate scandal and a key Cabinet member during the attempted assassination of President Reagan, he was a combat warrior who found himself a diplomat, a career military man who became the consummate political insider.

He died Saturday at Johns Hopkins Hospital in Baltimore from a staph infection he had before being admitted to the hospital, a Johns Hopkins spokeswoman said. He was 85.

Haig is probably best remembered by many Americans as the man who asserted his authority in the White House after Reagan was shot outside a Washington hotel in 1981 -- despite the fact that, as secretary of State, he was well down the line of presidential succession.

The gaffe sparked a small-scale firestorm. But Haig had been a controversial figure in Washington circles for years, chiefly for his role as President Nixon's chief of staff as the administration buckled beneath the weight of the Watergate investigation. Haig was credited by many for holding the White House together as its walls closed in on the president.

Haig, who attained the rank of four-star general, was that rare Washington species: a product of the armed forces who adapted seamlessly to the byzantine workings of executive power. Brash, steely, opinionated, he fought on the battlefield and navigated the corridors of the Pentagon and White House with equal aplomb, and his progress became linked with that of other notable figures of the period such as Douglas MacArthur, Robert McNamara, Henry Kissinger and, later, Nixon and Reagan.

So enmeshed was he in Nixon's inner circle that for years he was widely suspected of being the celebrated "Deep Throat," the anonymous source who fed reporters Carl Bernstein and Bob Woodward information on Watergate -- ultimately revealed to be FBI official W. Mark Felt.

After Nixon resigned, Haig returned to the military as the supreme commander of allied forces in Europe before Reagan tapped him as his first secretary of State, a post he occupied for just 18 months. The self-proclaimed "vicar" of American foreign policy, he struggled among the shifting turf battles of that administration's early years and soon became their victim.

He would unsuccessfully run for the Republican presidential nomination in 1988. Later in his career he turned his eye toward business, becoming an advisor to several corporations, serving as a founding board member of America Online, and commanding a hefty fee on the lecture circuit.

President Obama said Saturday in a statement that Haig "exemplified our finest warrior-diplomat tradition of those who dedicate their lives to public service."

Alexander Meigs Haig Jr. was born Dec. 2, 1924, in Bala Cynwyd, Pa., a suburb of Philadelphia. His father was a Republican lawyer who died of cancer when Haig was 10.

Raised Roman Catholic, Haig attended the University of Notre Dame before receiving an appointment to the U.S. Military Academy at West Point. He graduated in 1947 as the United States was settling into its new role as the global bulwark against communism.

He became a junior aide to Gen. MacArthur in Japan, where he met his wife, Patricia, the daughter of an Army officer. He had a tour of duty in Korea, serving in seven campaigns, including the Battle of Inchon, and was awarded two Silver Stars and a Bronze Star.

Such began an inside-outside pattern of advancement within the Army. After the Korean War ended in 1953, he served in a variety of administrative posts in both the U.S. and Europe, while also setting the stage for his future career by securing a master's degree in international relations from Georgetown University, where he studied the intersection of the military and policy.

His career began to move forward when he was appointed a top aide to Defense Secretary McNamara at the Pentagon, just as the Vietnam War was heating up. But Haig again sought the theater of combat and, as a lieutenant colonel, took command of a battalion of the 1st Infantry Division in Vietnam.

In 1967, his battalion was pinned down by the Viet Cong near Ap Gu. Haig flew to the scene in a helicopter, which was subsequently shot down. For three days, his soldiers, outnumbered 3 to 1, fought off waves of enemy troops, inflicting severe casualties. Haig was awarded the Distinguished Service Cross, the country's second-highest honor for heroism, by Gen. William Westmoreland.

Back in the States, his career took a life-changing turn when Haig, then a colonel, was appointed to the staff of President Nixon's national security advisor, Henry Kissinger. For four years, Haig ran Kissinger's office with military efficiency, impressing Kissinger and Nixon alike. He was dispatched to Paris to oversee the first round of peace talks with the North Vietnamese and later was part of the team that paved the way for Nixon's historic outreach to China.

But there were less savory endeavors: Haig also helped Kissinger and Nixon wiretap the phones of 17 people, including reporters and members of Kissinger's staff, suspected of involvement in leaks of government information.

Nixon, who placed loyalty and trust above all, wanted to keep Haig close and promoted him ahead of 240 other generals to vice chief of staff of the Army, the branch's No. 2 position, despite the fact that Haig had never commanded a division. Shortly thereafter, in May 1973, Nixon made him his chief of staff to replace H.R. Haldeman.

The president later wrote that he put Haig in charge of the White House because "what he might have lacked in political experience and organizational finesse he made up in sheer force of personality."