WASHINGTON (AP) — Harold Brown, who as defense secretary in the Carter administration championed cutting-edge fighting technology during a tenure that included the failed rescue of hostages in Iran, has died at age 91.

Brown died Friday, said the Rand Corp., the California-based think tank which Brown served as a trustee for more than 35 years. His sister, Leila Brennet, said he died at his home in Rancho Santa Fe, California.

Brown was a nuclear physicist who led the Pentagon to modernize its defense systems with weapons that included precision-guided cruise missiles, stealth aircraft, advanced satellite surveillance and improved communications and intelligence systems. He successfully campaigned to increase the Pentagon budget during his term, despite skepticism inside the White House and from Democrats in Congress.

That turbulent period included the Soviet Union’s invasion of Afghanistan and the Iranian hostage crisis. An effort in April 1980 to rescue the hostages failed when one of the helicopters on the mission struck a tanker aircraft in eastern Iran and crashed, killing eight U.S. servicemen.

“I considered the failed rescue attempt my greatest regret and most painful lesson learned,” Brown wrote in his book “Star Spangled Security.”

Brown faced numerous obstacles when he took the job as Pentagon chief, including pressure to reduce the defense budget both from within the administration and from influential congressional Democrats.

“When I became secretary of defense in 1977, the military services, most of all the army, were disrupted badly by the Vietnam War. There was general agreement that the Soviet Union outclassed the West in conventional military capability, especially in ground forces in Europe,” he wrote later.

Wary of the growing Soviet threat, Brown sought to withstand the pressure to cut defense and, gradually, managed to increase spending.

“The constant Cold War competition raged hot during the Carter administration and preoccupied me throughout the four years,” Brown wrote. He noted later that “the Defense Department budget in real terms was 10 to 12 percent more when we left than when we came in,” which he said was not an easy accomplishment.

And he cited the technological advances in defense systems, especially weapons systems such as precision-guided cruise missiles, stealth aircraft and advanced satellite surveillance.

“Some of these came to visible fruition 10 years later during Desert Storm, which reversed Saddam Hussein’s occupation of Kuwait,” he wrote. “The Carter administration initiated and developed these programs, the Reagan administration paid for their acquisition in many cases, and the George H.W. Bush administration employed them.”

Brown later maintained that his extensive work with the Soviets on the arms race was not wasted.

“We also reached a specific strategic arms control agreement with the Soviet Union,” he wrote. “Though never formally ratified, the agreement was adhered to by both parties and limited Soviet threats that our other conventional and nuclear weapons programs were designed to counter.”