Correction: An earlier version of this article incorrectly reported that Dr. Molander was trained as a physicist. He was trained in mechanical engineering and nuclear engineering.

Roger C. Molander, an arms control strategist who worked at the highest levels of government in the 1970s and later became a prominent grass-roots organizer after he grew convinced that policymakers alone could not avert a nuclear war, died March 25 at the Washington Home hospice in the District. He was 71.

The cause was complications from liver cancer, said his daughter Egan Molander Cammack.

Trained in nuclear engineering at the height of the Cold War, Dr. Molander spent his career trying to help the United States avert a nuclear war. He served on the National Security Council from 1974 to 1981 before starting a shoe-leather organization, Ground Zero, that sought to educate and mobilize the American public about the nuclear threat. He also co-wrote the 1982 primer “Nuclear War: What’s in it For You?,” which sold hundreds of thousands of copies.

He said the arms control community had become desensitized to the potential human costs of nuclear combat and came to believe that preventing a nuclear war required a broad movement from among the citizenry.

He once told the New York Times that a defining moment for him was a meeting at the Pentagon at which he claimed a Navy officer said that “people here and in Europe were getting much too upset about the consequences of nuclear war. The captain added that people were talking as if nuclear war would be the end of the world when, in fact, only 500 million people would be killed.’’

Roger C. Molander, 71, an arms control strategist who became a prominent grass-roots organizer after he grew convinced that policymakers alone could not avert a nuclear war, died March 25. (M. C. Valada/The Washington Post)

“Only 500 million people!” Dr. Molander recalled thinking. “I remember sitting there and repeating that phrase to myself: Only 500 million people! Only one-eighth of the world’s population!”

As a senior member of the National Security Council staff, Dr. Molander coordinated the work of Washington officials who supported the negotiators trying to reach an agreement with the Soviet Union on a second round of strategic arms limitation talks, known as SALT II. In that role, he won a reputation for fair-minded, rigorous staff work.

Dr. Molander “was theexpert on the NSC staff reporting to me on the complexities of that issue,” former national security adviser Zbigniew Brzezinski said in an interview. “Without him, we wouldn’t, I think, have been able to make the progress that we did.”

The first SALT talks, completed in 1972 when Dr. Molander was a Defense Department consultant, capped the number of nuclear launchers the two superpowers could maintain. The agreement that came from SALT II, in 1979, would have limited the number of nuclear warheads as well.

But shortly after the SALT II negotiations were successfully completed, the Soviet Union invaded Afghanistan. Relations between Washington and Moscow deteriorated, and the U.S. Senate refused to ratify the treaty.

That setback compounded Dr. Molander’s growing impatience about what he viewed as ignorance among government officials and average Americans about the extent of the danger of nuclear war.

“I remember looking at the growth in numbers and sophistication of weapons, the rejection of the treaty, the lack of prospects for meaningful arms control efforts and thinking: this is out of control,” he told the New York Times in 1982.

Dr. Molander started Ground Zero in 1981 with money from institutions such as the Rockefeller Family Fund. In April 1982, the group organized a week-long series of demonstrations across the country, including seminars, teach-ins and “dances against death,” that received national media coverage. The events attracted, as one reporter noted, people both in business suits and in bare feet.

Time magazine called Dr. Molander perhaps “the single most visible (and thoughtful) leader in the nebulous movement.”

With help from his twin brother, Earl Molander, he wrote “Nuclear War: What’s in it For You?,” a history of nuclear war and disarmament efforts.

Writing in Newsweek, Peter McGrath called it “an accessible book, providing the reader with the basic information needed to think and talk intelligently about nuclear war.”

But Washington Post columnist David Broder called the book an example of “liberal sentimentalism run amok.” Other critics pointed out that deterrence — long the dominant philosophy among national security policymakers — had been successful. The fact was, they argued, that the United States and the Soviet Union had not engaged in nuclear war.

From 1983 to 1989, Dr. Molander ran the Roosevelt Center for American Policy Studies, a Washington-based policy research institution. He then joined the Rand Corp. as a senior analyst and worked until shortly before his death on issues including nuclear proliferation and the repercussions of possible national disasters and pandemics on the intelligence community.

Roger Carl Molander was born Nov. 20, 1940, in Marinette, Wis. He received a bachelor’s degree in mechanical engineering from the University of Wisconsin in 1963 and a doctorate in engineering science and nuclear engineering from the University of California at Berkeley in 1967.

Survivors include his wife of 37 years, Mary Moore of Washington; two daughters, Egan Molander Cammack of Milton, Mass., and Ingrid Molander of Washington; his brother; and two granddaughters.

Despite a lifetime dedicated to preventing a nuclear holocaust, Dr. Molander considered himself an optimist. This was on evidence the day in 1982 when a Post reporter asked him why he was bothering with a major home renovation if nuclear annihilation was likely.

“I guess you could say it’s the ‘eat, drink and be merry, for tomorrow we die,’ sort of thing,” he said. “Who knows how long we’re here. I’d rather say that when you’re conscious of the dangers of nuclear war, you pay more attention to life.”