By PHILIP TAUBMAN


ALMOST FROM THE moment the first atomic bomb was detonated in New Mexico in July 1945, the menacing aura of the nuclear age has inspired visions of a world free of nuclear weapons. Never more so than now, with the prospect that the Taliban could someday control Pakistan’s nuclear weapons, North Korea might develop nuclear-tipped missiles, Iran may soon become a nuclear power and terrorists could get a bomb.

A growing army of nuclear abolitionists, concerned that proliferation could catch fire at any moment, is advancing the cause, led by Barack Obama, the first president to make nuclear disarmament a centerpiece of American defense policy.

Yet even as the allure of disarmament grows, the obstacles seem as daunting as ever. Going to zero, as the nuclear cognoscenti put it, is a deceptively simple notion; just about everyone who knows nuclear weapons agrees it would be wickedly difficult to achieve.

It would require vast changes on a wide array of issues, from core defense policies to highly technical weapons programs. To fully grasp the political and military implications, consider what would have been involved had the great powers of the 19th century decided to abolish gunpowder.

The new appeal of disarmament is driven by threats that in some ways make the nuclear equation more ominous and volatile than during the cold war, even though there are far fewer weapons now. As Mr.Obama said in April before a huge crowd in Prague: “In a strange turn of history, the threat of global nuclear war has gone down, but the risk of a nuclear attack has gone up.”

Nuclear conflict between the United States and the Soviet Union was a prospect so harrowing that leaders of both countries recognized it was untenable, even as their generals planned for Armageddon. They possessed some 70,000 nuclear warheads between them in the 1980s, but the weapons were under firm control and neither side dared risk the retaliation that a first strike would draw.

The dynamic today is much less stable, and more difficult for the United States to manage. As the number of nuclear-armed nations rises, the security of weapons and technology diminishes. Terrorists would have no compunction about using a nuclear weapon, and their target could not easily retaliate against an elusive, stateless group.

Faced with these dangers, Mr.Obama is banding with fellow leaders like President Dmitri A.Medvedev of Russia and Gordon Brown, the British prime minister, to push for steps to reduce nuclear threats in the near term, while preparing the ground for their eventual elimination.

The Obama administration and other advocates want the United States Senate to ratify the 1996 Comprehensive Test Ban Treaty; would strengthen the 1968 Nuclear Non- Proliferation Treaty; and would seek an accord to verifiably ban the production of fissile materials intended for use in weapons. It is an audacious agenda, but as alarm over nuclear threats rises, the chances of success seem to be growing.

How far can nuclear arms levels be reduced, short of abolition, while still providing deterrence- The United States and Russia announced last week in Moscow that they had agreed to cut the number of operational strategic warheads on each side by at least one-quarter, to between 1,500 and 1,675, from the present 2,200. Those numbers are generally deemed ample for deterrence. But the limit might have to go to 500 or fewer before nuclear weapons states with smaller arsenals, including China, would start cutting.

If nuclear arsenals are drastically reduced, the next steps toward abolition could be even trickier. Since scientific and engineering knowledge cannot be expunged from mankind’s memory, the potential to build weapons will always exist. Efforts to hide a few weapons may be difficult to detect and prevent. And any nation able to enrich uranium usable in nuclear power plants, like Iran, has a capacity to produce highly enriched fuel for weapons.

To address those issues, nuclear arms experts propose improvements in the tools used to monitor and verify compliance with treaties and new ways to prevent cheating, including more intrusive inspections.

The notion of nuclear disarmament gained credibility a few years ago when four cold war veterans - George Shultz and Henry Kissinger, former secretaries of state; William Perry, a former defense secretary; and Sam Nunn, the former chairman of the Senate Armed Services Committee - overcame their political differences to endorse the idea in a Wall Street Journal opinion article. Now that Presidents Obama and Medvedev have embraced it, the concept seems to be moving from the realm of fantasy to the world of policy and politics.

How far it goes may depend on how much world leaders and the public accept the proposition, as Mr.Nunn sometimes says, that “we are in a race between cooperation and catastrophe.”