Donald Trump might use nuclear weapons to go after Islamic State terrorists. Or maybe not. In a recent spate of interviews, including with The Times, he was unable or unwilling to clarify his disturbing views on this and other critical national security issues, which sometimes shift from one minute to the next.

The recent horrific terrorist attacks around the world have provided a new opportunity for Mr. Trump to fan fears and throw out his alarming prescriptions for dealing with the world’s most complex challenges. On Wednesday, Mr. Trump was asked if he would use tactical nuclear weapons against the Islamic State. “I’m never going to rule anything out — I wouldn’t want to say. Even if I wasn’t, I wouldn’t want to tell you that because at a minimum, I want them to think maybe we would use them,” he said on the Bloomberg Politics program “With All Due Respect.”

He was more measured in his comments to The Times on Friday, saying nuclear weapons are “the biggest problem the world has” and he would use such weapons only as “an absolute last step.” Even if Mr. Trump, the leading Republican presidential candidate, doesn’t really believe that nuclear weapons should be used against a terrorist group, the fact that he has voiced it lends weight to this insane notion and could make it easier for other nuclear-armed states to think about that possibility.

The consequences of using a nuclear weapon in terms of lives lost, physical destruction and cost to American moral standing would be devastating. The United States and Russia have significantly reduced their nuclear arsenals, and the threat that either would ever use the weapons has greatly receded, in part because advanced conventional weapons can destroy almost any military target. Equally bizarre was Mr. Trump’s casual attitude in endorsing the idea of Japan and South Korea developing their own nuclear weapons, which would reverse America’s longstanding efforts to prevent the number of nuclear-armed states from expanding.