The eyebrows of some were burned off and skin hung from their faces and hands. Others, because of pain, held their arms up as if carrying something in both hands. Some were vomiting as they walked. Many were naked or in shreds of clothing. On some undressed bodies, the burns had made patterns of undershirt straps and suspenders and, on the skin of some women (since white repelled the heat from the bomb and dark clothes absorbed it and conducted it to the skin) the shapes of flowers they had had on their kimonos. — John Hersey, “Hiroshima.”

WASHINGTON — It will be 64 years, in August, since that sunny morning when the Enola Gay dropped a small and crude atomic device, the “Little Boy,” on Hiroshima. A hundred thousand people, almost all civilians, died. The horror of that day, and the one that followed at Nagasaki, left a searing impression on humanity, even against the backdrop of the savagery that was World War II.

If the human race is fortunate — for we know it is not wise — the world will make it to 2045 without London, or New York, or Tehran, or Mumbai, or Tel Aviv, or Beijing suffering the fate of Hiroshima and Nagasaki. The odds for such a celebration were getting better, as the United States and Russia (which together have 95 percent of the world’s nuclear weapons) spent much of the past two decades reducing their arsenals. But now the day has come, of which we were warned, in which the prospect of losing a city to atomic flames is again increasingly likely.

In the last few weeks, Iran has reaffirmed its intention to develop a nuclear capability. North Korea tested a nuclear device last week, and test-fired ballistic missiles, and U.S. and South Korean armed forces were put on high alert. The nuclear-armed Pakistani state, meanwhile, continued with its war against Taliban extremists.

The actions of these nations alarm their neighbors, many of whom — Japan, South Korea, Saudi Arabia and Turkey to name a few — have the economic and technological resources to become nuclear powers themselves. “There is concern that we may be reaching a so-called nuclear tipping point,” said Charles Ferguson, a senior fellow at the Council on Foreign Relations.

And if the number of nuclear-armed states increases — to potentially as many as 30 or 40 nations — so will the likelihood that terrorists could acquire a nuclear weapon, or the enriched uranium needed to build such a device.

“Nuclear terrorism is a very serious threat,” said former Defense Secretary William Perry, at a Council on Foreign Relations forum Thursday. “It is the most likely way a nuclear bomb will end up being detonated in one of our cities.”

Former National Security Adviser Brent Scowcroft, sitting next to Perry, agreed. “We have been very lucky,” he said. With more and more nuclear-armed regimes “you have a tremendous danger of terrorists getting hold of fissile material, and then making a bomb is relatively easy.”

How likely? How lucky? How easy?

Last December, a commission of terrorism and weapons experts appointed by Congress and chaired by former Sen. Bob Graham completed the latest U.S. government report on the dangers posed by terrorists. If current trends continue, “it is more likely than not that a weapon of mass destruction will be used in a terrorist attack somewhere in the world by the end of 2013,” it concluded.

Though the commission found that terrorists were somewhat more likely to employ a biological, rather than nuclear, WMD in the next five years, it warned that the risks of nuclear terrorism were growing, not receding. And that was before the events of this spring.

“The number of states that are armed with nuclear weapons or are seeking to develop them is increasing,” the panel reported. “Terrorist organizations are intent on acquiring nuclear weapons or the material and expertise needed to build them. Trafficking in nuclear materials and technology is a serious, relentless and multidimensional problem.”

The White House is acutely aware of the danger. President Obama raised the issue of proliferation on the campaign trail last fall, and took time from dealing with the economic crisis to speak in Prague about nuclear dangers. His administration is negotiating with Russia over terms for a new strategic arms reduction treaty, to replace the bilateral agreement that expires next December. And on May 19, Obama met with the four “wise men” — former Republican Secretaries of State Henry Kissinger and George Schultz; former Democratic senator Sam Nunn and Perry — who have urged the world to embrace the goal of abolishing nuclear weapons.

“North Korea and Iran are in the process of developing nuclear weapons capacity,” Obama said afterward. “We see a country like Pakistan with a large nuclear arsenal on the other side of a long-running conflict in the subcontinent with India. Terrorist organizations like Al Qaeda are trying to seek fissile material.”

But though there is growing appreciation of the danger, it has not been accompanied by urgency and action. Reaching a new strategic arms deal with the Russians, and getting it ratified by the Senate, will be harder than it sounds, said Perry. Iran and North Korea have rebuffed American appeals and threats. Senate Republicans have told Sen. John Kerry, the chairman of the Foreign Relations Committee, that they will continue to block approval of the Comprehensive Test Ban Treaty — a modest pact designed to strengthen the international non-proliferation regime.

Scowcroft finds cause for hope in the many years that have passed since 1945, and the absence in that time of a conventional war on a scale approaching that of World War I or II. Nuclear weapons may have a stabilizing effect, he says. Even tyrants like Joseph Stalin and Mao Zedong were deterred by the prospect of U.S. nuclear retaliation.

But can the same be said about terrorists such as Osama bin Laden, or the Pakistani Taliban — whose leaders have publicly asserted a god-given authority to recruit women and children as suicide bombers?

“Pakistan poses one of the greatest dangers of nuclear weapons leaking out,” said Perry. “Anything we can do to maintain the stability of the government is important because the worst possible outcome is for an insurgent group dominated by the Taliban … controlling more than 100 nuclear weapons.”

The U.S. could not abide by that outcome, and would likely intervene militarily to keep Pakistan’s nuclear arsenal out of extremist hands. But even if that mission seemed to succeed, how could the U.S. be sure that no nuclear device, or enriched uranium, would make its way to terrorists? Would India sit by and risk that happening?

“The nuclear weapons at the present time, at least I’m confident, are secure,” said Scowcroft. “I think they’re secure as long as the Pakistani army is a unified cohesive force. If it isn’t then something else may need to be done.”

Americans rallied after 9/11. But the next time civilians fleeing a terrorist attack cross the Brooklyn Bridge from lower Manhattan, or stagger away from the Pentagon, they may fit the description that writer John Hersey gave of the victims at Hiroshima: “People screamed for help, but no one helped … for they could not comprehend or tolerate a wider circle of misery.” As many as 50,000 or 100,000 Americans could die in a nuclear attack by terrorists and, as rumors and reports of bombs in other cities careened around the nation, panic would set in, and trade and commerce shudder to a halt.

“Even if the threat were not true,” said Perry, “the chaos — the economic, the political, the social chaos — that would occur would be fantastic.”

More on nuclear proliferation:

Opinion: Time to encourage South Korea and Japan to go nuclear?

North Korea conducts second nuclear test

Obama proposes nuclear-free world

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