Peter D. Zimmerman is a nuclear physicist. He was a former chief scientist of the Senate Foreign Relations Committee, a former chief scientist of the U.S. Arms Control and Disarmament Agency and science adviser for arms control in the Department of State. He is a professor emeritus at King's College London and is based in Northern Virginia.

U.S. Ambassador to the U.N. Nikki Haley speaks during a Security Council meeting on North Korea on Monday. (Stephanie Keith/Reuters)

This month, the Treaty on the Prohibition of Nuclear Weapons will open for signature at the United Nations. Signatories will promise never to “develop, test, produce, manufacture … possess or stockpile nuclear weapons”; never to transfer weapons to other parties nor to receive them; and never to “use or threaten to use nuclear weapons.” The treaty’s aims, if they could be universally effected, are noble.

After all, the prospect of nations — including, now, an international pariah like North Korea — facing off with their respective nuclear arsenals is horrific. Their renewed use in war would be catastrophic. But there is a risk in aiming for total nuclear disarmament, because deterring nuclear war isn’t their only legitimate use. Nuclear weapons also deter conventional war.

In recent decades great powers have fought proxy wars, but since 1945 they have not come into direct armed conflict. Through the Cold War, nuclear weapons kept the peace in Western Europe. Only once, in the Cuban missile crisis, did deterrence come close to breaking, but President John F. Kennedy, Soviet Premier Nikita Khrushchev and the rest of the world learned well.

India and Pakistan have skirmished in recent decades, but the realization that a conflict could escalate to nuclear catastrophe has contributed to the rival nations eventually standing down. The probability that Israel has nuclear weapons is the ultimate guarantor of its existence.

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Since the Soviet Union’s first atomic test in 1949, the existence of nuclear weapons in many hands has not only deterred the use of nuclear weapons, but also made nuclear possessors and their adversaries think carefully about the desirability of going to war at all. When conflict has broken out, the nuclear deterrent has limited war aims to those short of total destruction of adversary nations or regime change. That’s why North Korea has sought nuclear capability so fervently.

This is the “porcupine” theory advanced by, among others, the late strategist Kenneth Waltz: After Hiroshima and Nagasaki, many states wanted nuclear weapons, not for offensive purposes, but as a hedge against attack by other nations. If peace is desirable, and it is, this seems, at first, philosophically unappealing; nonproliferation and nuclear elimination sound so much safer. But with obvious limits, this hedge has served as a practical solution to an intractable problem.

It’s a good thing, then, that the Treaty is probably going nowhere.

For starters, the nuclear powers aren’t on board: When negotiations concluded on July 7, 122 nations voted for the treaty. But the Netherlands, the only NATO country to participate in negotiations, voted against it. As CBS News notes, none of the countries “known or believed to possess nuclear weapons” — the United States, Russia, Britain, China, France, India, Pakistan, North Korea and Israel — support the treaty. Sweden was the only country with long-standing, close ties to NATO that voted for it. Most states voting for the treaty lack the capability, or significant interest, in acquiring nuclear weapons. If the proverbial mice decide to bell the cat, success will depend upon the cat’s consent to wear a bell.

And the agreement was rushed: Other international arms-control agreements, such as the prohibitions on chemical and biological weapons, on nuclear testing, and on strategic arms reduction have taken years and even decades to negotiate. The nuclear ban was completed after only a few days of negotiation in March and a few weeks in June and July. The treaty’s language is unhelpful. The preamble includes references to an assortment of humanitarian causes, bearing only the most tangential relationship to the topic at hand: “disproportionate impact of nuclear-weapon activities on indigenous peoples,” “disproportionate impact on women and girls,” and the role of the “Red Cross and Red Crescent Movement.”

Though “trust but verify,” as President Ronald Reagan often put it, remains the core of any international arms-control agreement, the U.N. treaty presents a nebulous mention that weapons states shall cooperate with a “competent international authority or authorities to negotiate and verify the irreversible elimination of nuclear weapons programs.” A “State Party that owns, possesses or controls nuclear weapons … shall immediately remove them from operational status” and later “submit to the Secretary-General of the United Nations a declaration that it has fulfilled its obligations.” The mechanics by which nuclear possessor states rid themselves of their weapons are undefined. For now, the agreement relies on the honor system, rather than enforceable penalties for noncompliance — critical details kicked down the road to a document that doesn’t yet exist.

[I’m a nuclear weapons expert. Trump’s presidency is my personal nightmare.]

Even if the document had been perfectly drafted, and had the leaders of the effort gained a measure of buy-in from nuclear states about their interests, total nuclear abolition remains a bad idea. As Margaret Thatcher said, 30 years ago, in a speech delivered in Russia:

“Conventional weapons have never been enough to deter war. Two world wars showed us that. They also showed us how terrible a war fought even with conventional weapons can be, yet nuclear weapons have deterred not only nuclear war but conventional war in Europe as well. A world without nuclear weapons may be a dream but you cannot base a sure defense on dreams. Without far greater trust and confidence between East and West than exists at present, a world without nuclear weapons would be less stable and more dangerous for all of us.”

The planet would be safer with far fewer nuclear weapons but more dangerous with none, and there is no way to prove that all such weapons have been eliminated.

Some hydrogen bombs are small enough to hide in a coat closet — verification of their destruction, in the absence of a yet-to-be-determined mechanism, and in the absence of a strong international consensus, is impossible. And the loss of the barrier to conventional escalation would be ruinous. Nuclear weapons cannot be un-invented. If the treaty’s proponents had their way, the world would eventually regret it.