In their natural state, atoms aren’t prone to catastrophic fracture. Nature abhors imbalance, and in stability lies strength. It is only when atoms are squeezed by intense external force that the bonds which bind them rupture. When they do, an extraordinary amount of energy is released from that critical mass, almost uncontrollably, in a mushroom cloud.

Armed with this knowledge, humans sought to unleash these devastating forces against their enemies. When the United States first used nuclear weapons against the blockaded, staggering remnants of the Japanese Empire, it was a destabilizing event. The Soviet Union rushed to purloin and perfect the technology, which it quickly did, bringing an equilibrium to a Cold War that lasted for a half-century. Those who sought power by riding the back of the tiger didn’t, as John F. Kennedy warned, end up inside — they simply found their own tigers to straddle.

That didn’t stop the development of bigger and better nuclear arms. “City-killing’’ passed to the hands of man, a power so awesome, so vast that it is almost impossible to articulate. The largest nuclear weapon ever developed was the Soviet Union’s RDS-220, also known as the Tsar Bomba. If it were detonated today directly over the city of Boston, it would destroy life between Manchester, N.H., and Provincetown, Portsmouth, N.H., and Providence, Worcester and Gloucester. All in a few seconds.

A test of the Tsar Bomba, in October 1961, so appalled one of its scientists, Andrei Sakharov, that he began to speak publicly against the existence of nuclear weapons, their spread, and their use. He later won a Nobel Peace Prize, and the European Parliament now awards the Sakharov Prize for Freedom of Thought annually.

The last 10 people to be awarded the Sakharov Prize would be prevented from entering the United States if Donald Trump wins the White House and stays true to his word to ban Muslims and those who come from nations that grapple with terrorism. There is no reason not to take the man at his word.

The Trump campaign has denied that the candidate asked one of his national security advisers three times during a briefing why the United States can’t use nuclear weapons. But such questions wouldn’t be surprising from a presidential candidate who has already refused to rule out the use of nuclear weapons in Europe and refused to rule them out in a fight against the martyrdom cult known as ISIS, if you take him at his word.

For a presidential candidate to simply utter those thoughts is profoundly destabilizing in a world that’s already being squeezed by powerful forces. The postwar period, for all its chaos and bloodletting, has been marked by a stability in the global order that 15,500 nuclear weapons today underpin. Ninety percent of those weapons belong to Russia and the United States.

Our delicate international equilibrium rests on decades of formal international agreements and treaties, all based on the informal principle that future use of these weapons is morally reprehensible. American security rests on a long-settled calculation that other nuclear powers won’t risk their own annihilation by launching a first strike. Foreign powers are secure when they firmly believe the same about us. Anything that shakes those convictions undermines American and global security.

Deterrence has been the arbor under which liberal democracy has flourished since the implosion of the Soviet Union. Led by human rights activists like Sakharov, it took root first in Lithuania, the first country to declare independence from the collapsing USSR in March 1990. More than a dozen democracies emerged from the wreckage of Soviet Communism, but they are still prisoners to geography. Ukraine and Georgia have both lost portions of their nations in recent years to an expansionist Russia committed to conquest by baby-step and avoidance of nuclear confrontation.

Trump was asked recently if he would defend Lithuania, Estonia, and Latvia — all NATO allies — against a Russian invasion. His answer will cost the American diplomatic community a small fortune in dental work, once they’ve retrieved their jaws from the floor. “Right now there are many countries that have not fulfilled their obligations to us,’’ he said, casting doubt on Washington’s commitment to the alliance it founded.

The precedent of abdicating treaty obligations bodes ill for the future of the global nuclear truce. If basic continuity is no longer a fundamental US strategic objective, what’s to stop other nations from misreading our intentions? If leaders are blindly reckless with their language to the point we can’t take them at their word, what does that say about the clarity of their thinking?

In the final analysis, Trump’s question about the use of nukes is frightening not only because he is said to have repeatedly posed it. It is frightening because the answer to the question is that nothing prevents the leader of any nuclear state from using nuclear weapons. All that’s needed to release the pent-up power of the atom in a nuclear explosion is someone willing to apply the necessary force.