STANFORD, Calif.

OVER the last three years, as I delved into the world of American nuclear weapons, I felt increasingly as though I had stepped into a time warp. Despite the nearly total rearrangement of the international security landscape since the demise of the Soviet Union in 1991, the rise of Islamic terrorism and the spread of nuclear materials and technology to volatile nations like Pakistan, North Korea and Iran, the Defense Department remains enthralled by cold war nuclear strategies and practices.

Barack Obama took office determined to change that. He has made progress on many fronts. Last week, he outlined a new, no-frills defense strategy, downsizing conventional forces. He now needs to double down on his commitment to refashion nuclear forces. He should trim the American nuclear arsenal by two-thirds to bring it down to a sensible size, order the Pentagon to scale back nuclear war-fighting plans so they are relevant to contemporary threats, remove most American intercontinental, land-based missiles from high alert and drop the quaint notion that a fleet of aging B-52 bombers can effectively deliver nuclear weapons to distant targets.

This agenda is not only desirable, it is doable without undercutting American security. It would save tens of billions of dollars a year, a relatively small amount by Pentagon standards, but every billion counts as Leon E. Panetta, the defense secretary, trims his budget. And the steps can safely be taken without requiring reciprocal moves by Russia that must be codified in a treaty.

For the last few months, the Obama administration has been conducting a classified review of the doctrines and operations that determine the shape and potential uses of America’s nuclear armaments. If the president pushes back against the defenders of the old order at the Pentagon and other redoubts of the nuclear priesthood, he can preserve American security while making the United States a more credible leader on one of today’s most critical issues — containing the spread of nuclear weapons. Like a chain smoker asking others to give up cigarettes, the United States, with its bloated arsenal, sounds hypocritical when it puts pressure on other nations to cut weapons and stop producing bomb-grade highly enriched uranium, the key ingredient of a crude nuclear weapon.