Fred Charles Ikle, 87, who served as the Reagan administration’s third-ranking official at the Defense Department, where he helped craft policy on arms control and nuclear strategy, died Nov. 10 at his home in Bethesda.

He had complications from a fall, said his daughter Judith Ikle.

A Swiss-born sociologist, Dr. Ikle (pronounced ee-clay) was an authority on nuclear arms control. In addition to positions at the Rand Corp., Harvard University and the Massachusetts Institute of Technology, he served in the Nixon and Ford administrations as director of the U.S. Arms Control and Disarmament Agency before Reagan tapped him to be undersecretary of defense for policy.

His tenure in the Reagan White House, from 1981 to 1988, made him one of the longest-serving members of the administration.

At the Defense Department, Dr. Ikle oversaw special operations, foreign arms sales and military assistance abroad, especially to right-wing rebel groups in Central America and to mujaheddin fighters in Afghanistan.

In an interview, former Defense Department official Richard Perle said Dr. Ikle had profound influence on Reagan and was considered an exclusive member of the commander in chief’s brain trust.

Dr. Ikle was an avowed opponent of the Cold War doctrine of “mutually assured destruction,” in which the potential of nuclear annihilation was considered so awful it would prevent nuclear war. He had once written that the concept “rests on a form of warfare universally condemned since the dark ages — the mass killing of hostages.”

Perle said that when Dr. Ikle first took office he became convinced that the Defense Department’s strategic war plan — the options presented to the president in the event of a nuclear attack — was “seriously flawed.”

Perle said that, at the time, the U.S. response to a Soviet attack involved a retaliatory strike on Moscow as well as attacks on Poland, Hungary and Czechoslovakia, which were formal allies of the Soviet Union through the Warsaw Pact.

“Fred thought that there was a very good chance that [if a war broke out] the Poles, Czechs and Hungarians would choose to fight on our side rather the Soviets’,” Perle said. “But if you bombed them all with atomic weapons, that possibility would be gone.”

Through carefully constructed arguments and dozens of papers, Dr. Ikle successfully changed the long-standing war plan, Perle said, to allow the Americans the option to withhold attacking those countries.

Perle said the policy change “had a lasting effect and also demonstrated that civilians could have a significant influence on war plans,” which had been considered the sacred responsibility of the uniformed military.

Fritz Karl Ikle was born Aug. 21, 1924, in Samedan, Switzerland. He attended the University of Zurich before he came to the United States after World War II. He changed his name to Fred Charles Ikle and became a U.S. citizen.

He received a master’s degree in 1948 and doctorate in sociology in 1950, both from the University of Chicago. His thesis work involved the social impact of bombing on cities during World War II.

For his research, he visited Dresden, Germany, the site of a devastating fire-bombing attack by the Allies, and Nagasaki, Japan, where the United States dropped an atomic bomb. His thesis became a book, “The Social Impact of Bomb Destruction,” published in 1958.

Before his government service, Dr. Ikle was a researcher at Rand and Harvard’s Center for International Affairs, and a political scientist at MIT. At Harvard, he met fellow faculty member Henry Kissinger, who as Nixon’s national security adviser recruited Dr. Ikle to join the administration in 1973.

For many years, Dr. Ikle served on the Pentagon’s Defense Policy Board and, until his death, was a researcher at the Center for Strategic and International Studies, a public policy group in Washington.

Survivors include his wife of 51 years, Doris Eisemann Ikle of Bethesda; two daughters, Judith Ikle of San Francisco and Miriam Ikle-Khalsa of Takoma Park; and three grandchildren.

Colin Powell, the former chairman of the Joint Chiefs of Staff, wrote in his autobiography that Dr. Ikle’s 1971 book “Every War Must End” — about how the military should prepare a war’s exit strategy in advance — influenced his decision for a quick drawdown of the Persian Gulf War of the early 1990s.

In a 2006 Foreign Affairs review of the book, British war historian Sir Lawrence D. Freedman concluded that Dr. Ikle’s work demonstrated “how with clear prose, broad knowledge, and a sharp focus, a little book can address a big question.”

Freedman noted that Dr. Ikle’s ideas maintained their relevance more than 40 years since the book’s original publication.

“Cutting one’s losses, although a common notion in everyday life,” Dr. Ikle wrote in the book, “appears to be a particularly difficult decision for a government to reach in seeking to end a prolonged and unsuccessful war.”