The policy of every American president since Harry S. Truman has been to limit the proliferation of nuclear weapons.

They have not always been successful. The world’s most powerful weapons spread, with nine countries now possessing them. But no United States president has actively sought their further proliferation, as the belligerent policies of Donald Trump are now set to do.

In 2018, during his first term as president, Trump tore up the Iran nuclear deal, which had successfully placed limits on the enrichment of weapons-grade materials in exchange for sanctions relief.

Iran has since accelerated its nuclear weapons program. Estimates now put Iran within months or even weeks of producing several bombs.

A short time later, after a series of escalating threats, Trump suggested that North Korea had agreed to denuclearize. Talks ensued, but a deal never materialized.

In fact, Trump failed to stop, let alone roll back, North Korea’s ambitious nuclear weapons programs. North Korea is now said to possess at least 50 warheads as well as the means to deliver them.

Under the second Trump administration, the world is facing a rapidly growing proliferation risk of a different kind, one that is found not only among the usual suspects in Iran and North Korea, but also among a long list of U.S. allies who once basked in American security guarantees.

Merely two months into Trump’s second term, America’s European allies have grown increasingly concerned that the U.S. is no longer a reliable ally.

That’s due to his suspension (and then reinstatement) of weapons transfers and intelligence sharing with........

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