WASHINGTON— The mighty U.S. arsenal of nuclear weapons, midwived by World War II and nurtured by the Cold War, is declining in power and purpose while the military's competence in handling the world's most dangerous arms has eroded.

At the same time, international efforts to contain the spread of such weapons look ineffective.

Defense Secretary Robert Gates, for one, wants the next president to think about what nuclear middle-age and decline means for national security.

Gates joins a growing debate about the reliability and future credibility of the American arsenal with his first extensive speech on nuclear arms Tuesday. The debate is attracting increasing attention inside the Pentagon even as the military is preoccupied with fighting insurgencies in Iraq and Afghanistan. The unconventional tools of war there include covert commandos, but not nuclear weapons.

Gates is expected to call for increased commitment to preserving the deterrent value of atomic weapons. Their chief function has evolved from first stopping the Nazis and Japanese, then the Soviets. Now the vast U.S. stockpile serves mainly to make any other nation think twice about developing or using even a crude nuclear device of its own.

The chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, wrote in the current issue of an internal publication, Joint Force Quarterly, that the United States is overdue to retool its nuclear strategy. He referred to nuclear deterrence — the idea that the credible threat of U.S. nuclear retaliation is enough by itself to stop a potential enemy from striking first with a weapon of mass destruction.

"Many, if not most, of the individuals who worked deterrence in the 1970s and 1980s — the real experts at this discipline — are not doing it anymore," Mullen wrote. "And we have not even tried to find their replacements."

Gen. Kevin Chilton, commander of U.S. Strategic Command, which is responsible for maintaining the nation's nuclear war plans, told Congress last spring that technical nuclear expertise also is lagging.

"The last nuclear design engineer to participate in the development and testing of a new nuclear weapon is scheduled to retire in the next five years," Chilton said.

Of the two senators competing to succeed President Bush, Democrat Barack Obama is most unequivocally against building new nuclear weapons. Both he and Republican John McCain say in their campaign materials that they support the long-standing U.S. commitment to eventually do away with nuclear arms. Neither says explicitly that the safety or credibility of the arsenal is in question; that's an argument made most frequently by congressional Republicans.

Sen. John Kyl, R-Ariz., for example, said in a speech Sept. 15 that the network of laboratories and industrial plants that produce and maintain U.S. nuclear weapons is, in some cases, "simply falling down from age," and that this amounts to an alarming national "emergency."

Some private experts dispute Kyl's assessment.

"It's completely overblown," said Hans M. Kristensen, who tracks nuclear weapons developments for the Federation of American Scientists. The advocacy group opposes the Bush administration's proposal to develop a new nuclear weapon design.

The number of nuclear weapons in the U.S. arsenal is a state secret. But Kristensen and a fellow expert, Robert S. Norris, estimate that the total stood at nearly 5,400 warheads at the start of this year. That includes an estimated 4,075 ready for potential use and 1,260 in backup status.

In an interview, Kristensen argued that even though the number is declining, the capability of remaining weapons is increasing as older missiles, for example, get new engines, guidance sets and computer software.

Gates takes a different view. He has expressed concern about lack of official attention to the nuclear arsenal.

"Even though the days of hair-trigger superpower confrontation are over, as long as other nations possess the bomb and the means to deliver it, the United States must maintain a credible strategic deterrent," he said Sept. 29 in a speech at the National Defense University.

Gates tied the question of credibility to well-publicized slip-ups in Air Force nuclear operations. In June he fired the Air Force's top general, Michael Moseley, as well as the top civilian, Michael Wynne, after an outside investigation concluded that the Air Force had not adequately heeded warning signs that its nuclear expertise, performance and stewardship were eroding over a period of years.

In August 2007, a B-52 bomber flew from an Air Force base in North Dakota to a base in Louisiana with nuclear warheads that neither the bomber's pilots nor its crew knew were aboard. Then came the revelation that electrical fuses that trigger the detonation of strategic nuclear missiles had been shipped mistakenly to Taiwan — and the mistake was not discovered for months.

Richard Wagner, a physicist who worked in the government's nuclear weapons laboratories for many years, told a conference in Washington this past week that the August 2007 incident was "the worst breach of security of nuclear weapons that the United States has ever had."