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.

Last week, Mr. Obama was mired in the gritty business of trying to coax Pakistan’s president, Asif Ali Zardari, and the Afghan president, Hamid Karzai, into a more cooperative relationship and a more determined fight against the Taliban and Al Qaeda. Administration officials did not sound sanguine about the prospects, and the White House meeting might well have left Mr. Obama yearning for a more promising long-term strategy to keep the Taliban away from nuclear weapons.

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.