IS there any scarier nightmare than President Donald J. Trump in a tense international crisis, indignant and impatient, with his sweaty finger on the nuclear trigger?

“Trump is a danger to our national security,” John B. Bellinger III, legal adviser to the State Department under President George W. Bush, bluntly warned.

Most of the discussion about Trump focuses on domestic policy. But checks and balances mean that there are limits to what a president can achieve domestically, while the Constitution gives a commander in chief a much freer hand abroad.

That’s what horrifies America-watchers overseas. Der Spiegel, the German magazine, has called Trump the most dangerous man in the world. Even the leader of a Swedish nationalist party that started as a neo-Nazi white supremacist group has disavowed Trump. J. K. Rowling, author of the Harry Potter books, reflected the views of many Britons when she tweeted that Trump is worse than Voldemort.

Leading American conservative thinkers on foreign policy issued an open letter a few days ago warning that they could not support Trump. The signatories include Michael Chertoff, the former secretary of homeland security, Robert Zoellick, the former deputy secretary of state, and more than 100 others.

“Mr. Trump’s own statements lead us to conclude that as president, he would use the authority of his office to act in ways that make America less safe,” the letter declared.

A starting point is Trump’s remarkable ignorance about international affairs. And every time he tries to reassure, he digs the hole deeper. Asked in the latest debate to name people whose foreign policy ideas he respects, Trump offered Gen. Jack Keane, and mispronounced his name.

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