Alexander M. Haig Jr., the four-star general who served as a confrontational secretary of state under President Ronald Reagan and a commanding White House chief of staff as the Nixon administration crumbled, died Saturday at the Johns Hopkins Hospital in Baltimore, according to a hospital spokesman. He was 85.

Mr. Haig was a rare American breed: a political general. His bids for the presidency quickly came undone. But his ambition to be president was thinly veiled, and that was his undoing. He knew, Reagan’s aide Lyn Nofziger once said, that “the third paragraph of his obit” would detail his conduct in the hours after President Reagan was shot, on March 30, 1981.

That day, Secretary of State Haig wrongly declared himself the acting president. “The helm is right here,” he told members of the Reagan cabinet in the White House Situation Room, “and that means right in this chair for now, constitutionally, until the vice president gets here.” His words were taped by Richard V. Allen, then the national security adviser.

His colleagues knew better. “There were three others ahead of Mr. Haig in the constitutional succession,” Mr. Allen wrote in 2001. “But Mr. Haig’s demeanor signaled that he might be ready for a quarrel, and there was no point in provoking one.”