Making matters worse, Mr. Trump, in a separate decision on Friday, continued to put the 2015 deal that froze Iran’s nuclear program in jeopardy. The president warned European allies that they must agree to overhaul the deal in 120 days, or he would withdraw the United States from it. Although he again stopped short of reimposing sanctions, his demands would effectively require renegotiating the deal, something the other parties to the agreement have refused to do.

The proposed nuclear policy says a more aggressive nuclear posture is warranted because the world is more dangerous, with China, North Korea and Iran cited as concerns. Yet blowing up the Iran deal would free Tehran to resume its nuclear activities and make the world less safe. In other words, Mr. Trump’s approach makes no sense.

Under the Nuclear Nonproliferation Treaty, signed in 1968, the United States and Russia promised to reduce the role and number of nuclear weapons. They made significant, although insufficient, progress. After reductions under a succession of past presidents, the American stockpile is 85 percent smaller than it was at the height of the Cold War. Negotiations on further reductions have stalled in recent years as Russia, threatened by America’s superior conventional arsenal, became more reliant on nuclear weapons, and there is no serious sign that Mr. Trump wants to revive the talks.

President Barack Obama made a down payment on a saner policy by narrowing to “extreme circumstances” the conditions under which nuclear weapons would be used and ruling out their use against most non-nuclear countries. Mr. Trump’s policy also talks about “extreme circumstances, ” but it dangerously broadens the definition to include “significant non-nuclear strategic attacks,” which could mean using nuclear weapons to respond to cyber, biological and chemical weapon attacks.

Until Mr. Trump, no one could imagine the United States ever using a nuclear weapon again. America’s conventional military is more than strong enough to defend against most threats. But Mr. Trump has so shaken this orthodoxy that Congress has begun debating limits on his unilateral authority to launch nuclear weapons. Expanding the instances when America might use nuclear weapons could also make it easier for other nuclear-armed countries to justify using their own arsenals against adversaries.

As the residents of Hawaii can tell you, it’s a risk the world cannot afford.