It's the ultimate nightmare. Somewhere on the planet, a nuclear weapon is used for the first time since 1945. A fleet, army, military base — or a city — disappears in a radioactive mushroom cloud and thousands are killed. The 74-year taboo against using nuclear weapons has just been broken in a big, big way.

The Obama administration is reportedly pondering a fundamental shift in the nation's nuclear policy, one that will make sure that weapon isn't dropped by the United States. The Obama administration is reportedly considering an official policy of No First Use, meaning that the United States would never be the first party in any conflict to break the nuclear taboo.

Love them or hate them, nuclear weapons have been a cornerstone of American defense policy since 1945. Always seeking the technological edge, the U.S. invented nuclear weapons as a way to massively increase the amount of destruction it could unleash. Their use was hoped to end the war against the Axis, and arguably it did.

Unfortunately the American monopoly on nuclear weapons was short-lived, and 72 years later nine states have The Bomb: the United States, Russia, China, the United Kingdom, France, Pakistan, India, Israel, and North Korea. Globally, there are about 15,500 nuclear warheads. The United States has 1,481 nuclear weapons on active duty, with another 3,000 in storage and 2,500 waiting to be dismantled.

During the Cold War, the Pentagon planned to use so-called tactical nuclear weapons on the battlefield on a broad scale, in order to help outnumbered G.I.s on the ground. At the time, the idea that the U.S. would use nuclear weapons first in any conflict was a distinct possibility — as was the likelihood that such use would spiral out of control, into all-out nuclear warfare.

Today, America has a clear advantage over almost all potential adversaries in conventional weapons and technology. The prospect of American troops needing nuclear backup is unlikely. Still, Washington implicitly reserves the right to use nuclear weapons first in any conflict. It hasn't said it would use them first, but hasn't said it would wait until someone else used them, either.

Under the concept of No First Use, the United States would retain its nukes. It would be a continuation of the practice of nuclear deterrence that kept nuclear war from breaking out during the Cold War. Our weapons would still be available to retaliate against any country that used them first — likely in an overwhelming attack that would completely destroy the aggressor as a civilization. A first strike against America would still be suicidal.

Advocates of No First Use say that it would do a lot to reduce the risk of nuclear warfare. Bound by a pledge to not launch first, the U.S. could shift its nukes to a lower state of readiness. The risk of accidental nuclear war would decrease, and states without nuclear weapons would have less incentive to build them.

What would America give up by shifting to the new policy? Not much. Given our superiority in conventional weapons, it's difficult to imagine a situation in which America would have to resort to nuclear weapons before someone else does. Where other countries might consider using them, our enormous arsenal of tactical aircraft, combat ships, drones, tanks, and infantry is available instead. No First Use would not affect our conventional forces — which might even be expanded to give the U.S. government a greater range of non-nuclear options in a crisis.

Critics of No First Use argue that just because it's difficult for us to imagine a situation where we would need to use them first, that doesn't mean our enemies won't present us with one. A possible example: A North Korean nuclear missile, buried in a silo deep underground, is being prepared for launch. The silo is too well protected by concrete and steel to guarantee a conventional bomb — or hail of bombs — would destroy the missile. On the other hand, the newly updated B61-12 ground-penetrating nuclear bomb might very well do the trick.

Under No First Use, the Pentagon would have to drop conventional bombs and, if that fails, try to shoot the missile down. Either might succeed, but either might fail. Under the current policy, the president of the United States could at least consider the nuclear option.

Critics also point out that nuclear deterrence works against rational actors, but the world is filled with irrational ones. The leaders of the Soviet Union and China during the Cold War were rational players who understood the consequences of nuclear warfare. Modern adversaries, including the theocracy running the Islamic Republic of Iran and dictators such as Kim Jong Un, often do things that are less than rational, or even erratic.

Adoption of a nuclear No First Use policy could be a noble — and harmless — step that could lead to eventual worldwide nuclear arms reductions and lessen the risk of accidental nuclear war. It could also bar American military commanders from the most effective solutions to very serious problems. Whatever President Obama decides, it will have lasting repercussions on American security for decades to come.