Harold Brown, a brilliant scientist who helped develop America’s nuclear arsenal and negotiate its first strategic arms control treaty, and who was President Jimmy Carter’s secretary of defense in an era of rising Soviet challenges, died on Friday at his home in Rancho Santa Fe, Calif. He was 91.

His daughter Deborah Brown said the cause was pancreatic cancer.

As defense secretary from 1977 to 1981, Mr. Brown presided over the most formidable power in history: legions of intercontinental ballistic missiles and fleets of world-ranging bombers and nuclear submarines, with enough warheads to wipe out Soviet society many times over. But that was hardly the question.

In an age that imperiled humanity with nuclear Armageddon, the issue was whether America could keep pace with Soviet strategic capabilities, maintaining the balance of terror — an assurance of mutual destruction, with hundreds of millions killed outright — that had dominated the nuclear arms race and strategic planning throughout the postwar era.

In those days, “Dr. Strangelove,” Stanley Kubrick’s 1964 black comedy film about the doctrine of mutually assured destruction, shaded debates over nuclear strategy because the concept of deterrence was based on the dubious assumption that if the Russians launched a surprise nuclear attack, America could survive and retaliate, devastating Soviet cities and strategic targets, although millions would die.