Canadians are openly discussing the merits and risks of pursuing a nuclear weapon. Europeans are similarly considering a nuclear deterrent for the bloc. In South Korea, public support for a nuclear weapon is at its highest level on record, and even in Japan some politicians are talking about the once-taboo subject.

Until just a few years ago, few experts would have predicted that these nations – all allies of Washington – might one day join the nuclear club. Since 2006, that club has consisted of just nine countries: the United States, Russia, the United Kingdom, France, China, India, Pakistan, North Korea and Israel, with its undeclared program.

The hope of nonproliferation advocates was that nine would be the maximum. But over the past few years, more and more nations are seriously exploring “going nuclear.”

As an expert on weapons technology and former assistant director of the CIA for weapons and counterproliferation, I have watched these developments with alarm.

Perceived national security threats still shape U.S. allies’ views of developing nuclear weapons – with North Korea a key driver for South Korea, China paramount for Japan, and Iran key for Saudi Arabia.

But what has changed demonstrably for many U.S. allies is a newfound skepticism over the credibility of the so-called U.S. nuclear umbrella, which for decades has offered allies an easy way of declining to pursue nuclear weapons. Concerned about the Trump administration’s foreign policy, some nations are considering developing domestic nuclear weapons programs or seeking new deterrence assurances.

New nuclear aspirants across the Atlantic

The U.S. deployed the first atomic bombs in Japan in 1945, with the Soviet Union conducting its own successful nuclear test four years later. The U.K. was next to get the bomb in 1952, followed by France in 1960 and China in 1964. Experts believe that Israel first tested a bomb in the late 1960s, while the last entrants into the nuclear club were India in 1972, Pakistan in 1998 and North Korea in 2006.

Experts have long wondered which country might be next. Often, speculation has included U.S. allies such as Egypt, Japan, Saudi Arabia, South Korea and Turkey.

For years, such nations were assumed to be under the U.S. nuclear umbrella, a tacit understanding that........

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