David A. Andelman, visiting scholar at the Center on National Security at Fordham Law School and director of its Red Lines Project, is a contributor to CNN where his columns won the 2017 Deadline Club Award for Best Opinion Writing. Author of "A Shattered Peace: Versailles 1919 and the Price We Pay Today," he was formerly a foreign correspondent for The New York Times and CBS News in Asia and Europe. Follow him on Twitter @DavidAndelman . The views expressed in this commentary are his own.

(CNN) From somewhere above 31,000 feet, heading to his North Korean summit in Singapore, Donald Trump pitched a hand grenade into this weekend's G-7 meeting of the world's leading democracies.

In a fit of pique over remarks by the G-7 host, Canadian Prime Minister Justin Trudeau, Trump reversed his decision to sign a closing communiqué carefully crafted by his six fellow members, tweeting from Air Force One , "I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!"

This last-minute reversal raises a dangerous question: How can Kim Jong Un trust that any agreement he makes with Trump would last beyond the setting of the next sun?

Of course, in Singapore, we will have perhaps the only two world leaders congenitally incapable of keeping any deal they've struck.

Still, G-7 2018 is in the books. The leaders of the other six leading democracies in attendance have now scattered to their homes. Yet the wounds Trump has left behind are deep and will not soon be forgotten.

So, what exactly happened along the idyllic shores of the Saint Lawrence River in a remote stretch of Quebec ? The leaders of the developed world gathered for what promised from the outset to be a weekend of invective and calumny -- deepening conflict with Trump's America. Never mind that for much of the existence of this gathering , which dates back nearly a half a century, the United States served as the beacon of the world's leading democracies.

Outwardly, there were the usual smiles and handshakes, the first-name congeniality as the presidents, prime ministers and a chancellor assembled for their family photo and mugged for the cameras at photo-ops and preprandial gatherings. In the end, all seven thought they'd managed to paper over their differences with a joint communique signed by each country, including the United States.

But at his closing news conference, Trudeau suggested that he and others at the summit will hardly cave to Trump's tariff demands, adding that he found some of Trump's language "insulting" and that "we will not be pushed around."

On Sunday morning, Trump's chief economic adviser Larry Kudlow said on CNN's "State of the Union" that Trudeau's remarks were a "sophomoric political stunt." So to make sure, as Kudlow continued, that Trump did not "permit any show of weakness on the trip to negotiate with North Korea," the president retaliated in kind -- pulling a stunt that could backfire badly.

The big question: Are stunts the stuff of good diplomacy, or the way friends and allies should treat each other?

Indeed, the knives were not concealed very carefully at all. As Trump bolted to avoid the most uncomfortable moments of the gathering -- debate over global warming, which he utterly rejects -- he left behind a bad taste everywhere he turned.

Arriving late to the get-together, he missed his chance for a substantive meeting with French President Emmanuel Macron, who had been his one friend until a tense phone call earlier in the week. Then, he appeared late for the breakfast to discuss gender equality the next morning.

How could it possibly get any worse? Well, the President offered a few hints. He could, it seems, simply end all trade with any of their nations if they don't stop what he defines as unfair trade practices. Or he could continue to press for the pariah state of Russia, which Trump may not even have realized had been ousted after it seized control of Crimea from Ukraine, to return to the group.

Even today, Russia, still unrepentant for this action, would be the only autocracy in the G-8, with a GDP that ranks below every other member and indeed below non-members Brazil, India, China and South Korea.

If much of diplomacy is optics, President Trump seems to have left his opera glasses at home. Why would any world leader want to reward this kind of bad behavior by arriving at a quick trade deal that will only enshrine threats as the standard for accomplishing a purpose that should be the carefully considered outcome of discussion and diplomacy?

In one fashion or other, it seems, most of the other G-7 leaders have felt betrayed. All these leaders believe deeply in the natural order of things and the value of preserving a global club of democracies, while Trump believes -- and has never demonstrated more clearly than in Charlevoix -- in simply shredding such values at will and for his own convenience or profit. This weekend's G-7 simply placed all these feelings in greater, starker relief.

Ironically, it appears increasingly as though Trump is more comfortable and less threatened when dealing with autocrats or dictators: Welcome Putin back to the G-8; jet halfway around the world to spend some quality time with Kim Jong Un, a leader ruthless to all, even his own family

The question, which must remain unanswered, is whether this weekend has left America a step closer to embracing or professing preference for autocracy itself. Certainly, with Trump leaving an empty chair Saturday afternoon as the G-7 discussed climate change, sustainable energy and renewable fuels, America has abdicated its onetime leadership position on these critical issues.

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The realization has finally dawned that Donald Trump does not respond to rhetoric or reason -- or to anyone ganging up on him. The leading French daily Le Monde observed that the G-7 is simply "a symbol of a multilateral cooperation which he abhors." He does respond, however to strength and spine.

So, the battle was not joined in Charlevoix. But it will likely be joined in the weeks to come in retaliation, or at least with fortitude, as the G-6 unite against what now appears quite clearly to be their declared and sworn enemy.