Is there any scarier nightmare than President Donald 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, a legal adviser to the US State Department when George W. Bush was president, bluntly warned.
I can be more presidential than anybody: Trump
RAW VIDEO: Republican front-runner Donald Trump sweeps to victory in the Mississippi and Michigan presidential primaries, deepening his grip on the GOP nominating contest.
Most of the discussion about Trump focuses on US domestic policy. But checks and balances mean 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 they could not support Trump.
"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, with more than 100 signatories, 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 presidential debate to name people whose foreign policy ideas he respects, Trump offered General Jack Keane, and mispronounced his name.
Asked about Syria in 2015, Trump said he would unleash Islamic State to destroy Syria's government. That is insane: IS is already murdering or enslaving Christians, Yazidis and other religious minorities; executing gays; destroying antiquities; oppressing women. And Trump wants IS to capture Damascus?
A second major concern is that Trump would start a trade war, or a real war. Trump told The New York Times in January thathe favoured a 45 per cent tariff on Chinese goods – which would risk a trade war between the world's two largest economies.
Trump has also called for more US troops on the ground in Iraq, and raised the prospect of bombing North Korean nuclear sites. A poorly informed, impatient and pugnacious leader can cause devastation, and that's true of either Kim Jong Un or Donald Trump.
The third risk is to America's reputation and soft power. Both Bush and President Barack Obama worked hard to reassure the world's 1.6 billion Muslims the US is not at war with Islam. Trump has pretty much declared war on all Muslims.
The damage to the country's image is already done, even if Trump is never elected. Simply as a blowhard who gains headlines around the world, he reinforces caricatures of the US and tarnishes its global reputation. He turns America into an object of derision. He is America's Ahmadinejad.
The damage to America's image is already done, even if Trump is never elected. Simply as a blowhard who gains headlines around the world, he reinforces caricatures of the US and tarnishes its global reputation.
On Twitter, I suggested Trump was pugnacious, pugilistic, preening and puerile, and asked for other P words to describe him. The result was a deluge: petulant, pandering, pathetic, peevish, prickly, pernicious, patronising, phoney, presumptuous, potty-mouthed, provocative, pompous, predatory and so many more, including the troubling "probably president".
There's something heartbreaking about the prospect the next US president might be a global joke, a man regarded in most foreign capitals as a buffoon, and a dangerous one.
Trump is not particularly ideological, and it's possible that as president he would surround himself with experts and would back off extreme positions. It was a good sign that last week he appeared to reverse himself and pledged that he would not order the US military to commit war crimes, yet that's such an astonishingly low bar that I can't believe I just wrote this sentence.
In any case, Trump is nothing if not unpredictable, and it seems equally plausible he would start new wars. It's a risk few sensible people want to take. As Mitt Romney notes, "This is the very brand of anger that has led other nations into the abyss".
Peter Feaver, a political scientist who was a national security official in the Bush White House , noted that most Republicans are united in believing Obama and Hillary Clinton have damaged the US and added to the burdens of the next president.
"Yet, what Trump promises to do would in some important ways make all of the problems we face dramatically worse," he told me. "Why, at a moment when the country desperately needs our A team, would we send in the clowns?"
Nicholas Kristof is a columnist with The New York Times.