Eighty years ago in the Japanese cities of Hiroshima and Nagasaki, more than 200,000 people were killed in ways that nearly defy imagination: incinerated, burned alive, boiled in rivers, or slowly consumed by radiation sickness. Over the past decades, the international community has attempted to establish safeguards against nuclear proliferation and the use of nuclear weapons, a mission best articulated by U.S. President Ronald Reagan, who said that a nuclear war “cannot be won and must never be fought.”

Today, however, with resurgent nuclear brinkmanship and proliferation, it is easy to wonder whether developments in international law matter much at all, let alone enough to stem a nuclear launch. Both the United States and Russia have withdrawn from the Intermediate Range Nuclear Forces Treaty. Veiled nuclear threats are increasing between Russia and the United States, and Saudi Arabia has signaled an interest in arming. Even non-nuclear states in Europe, fearful of losing access to a U.S. nuclear umbrella with U.S. President Donald Trump pulling back from NATO, have begun considering acquiring nuclear weapons themselves. It is no wonder that Doreen Horschig and Heather Williams argued, even before Trump returned to office, that the nuclear order is “crumbling.”

Eighty years ago in the Japanese cities of Hiroshima and Nagasaki, more than 200,000 people were killed in ways that nearly defy imagination: incinerated, burned alive, boiled in rivers, or slowly consumed by radiation sickness. Over the past decades, the international community has attempted to establish safeguards against nuclear proliferation and the use of nuclear weapons, a mission best articulated by U.S. President Ronald Reagan, who said that a nuclear war “cannot be won and must never be fought.”

Today, however, with resurgent nuclear brinkmanship and proliferation, it is easy to wonder whether developments in international law matter much at all, let alone enough to stem a nuclear launch. Both the United States and Russia have withdrawn from the Intermediate Range Nuclear Forces Treaty. Veiled nuclear threats are increasing between Russia and the United States, and Saudi Arabia has signaled an interest in arming. Even non-nuclear states in Europe, fearful of losing access to a U.S. nuclear umbrella with U.S. President Donald Trump pulling back from NATO, have begun considering acquiring nuclear weapons themselves. It is no wonder that Doreen Horschig and Heather Williams argued, even before Trump returned to office, that the nuclear order is “crumbling.”

It is true that the nonproliferation regime is under strain. When scholars refer to this regime, they are typically talking about the Nuclear Non-Proliferation Treaty (NPT), which 191 states—including the five original nuclear ones—have signed and ratified. The NPT permits the five to possess nuclear weapons on the condition that they will work together to prevent others from obtaining them and take steps toward disarmament; non-nuclear states pledged not to acquire the weapons in exchange for the right........

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