James Stavridis is a Bloomberg Opinion columnist. He is a retired U.S. Navy admiral and former supreme allied commander of NATO, and dean emeritus of the Fletcher School of Law and Diplomacy at Tufts University. He is also an operating executive consultant at the Carlyle Group and chairs the board of counselors at McLarty Associates. Read more opinion LISTEN TO ARTICLE 4:46 SHARE THIS ARTICLE Share Tweet Post Email

Photographer: Saul Loeb/AFT/Getty Images Photographer: Saul Loeb/AFT/Getty Images

Over the past few days, our nation’s volatile president attended two summits -- one in Canada with America’s closest allies, friends and partners; and the other in Singapore with a brutal dictator.

Presented with these two incredibly diverse sets of interlocutors across the table, Donald Trump chose to call Canadian Prime Minister Justin Trudeau “weak and dishonest,” while calling Kim Jong Un, arguably the worst leader in the world in terms of human rights, someone “very smart” and an “honorable” man whom he intends to invite to the White House.

Beyond the basic theater of the absurd on display here, there is real damage to the entire structure of the trans-Atlantic alliance and NATO -- of which six of the G-7 nations are members -- that is deeply concerning.

When I was supreme allied commander of NATO, I attended several summits as an adviser to a very different sort of president, Barack Obama, and his secretaries of defense. I briefed the assembled heads of state on alliance operations in Afghanistan, the Balkans, Libya and fighting piracy off the coast of Africa, all part of my responsibility. I watched the intimate personal relationships between these democratically elected leaders grow stronger over time. While there was certainly plenty of drama and disagreement, it was kept off the public record and settled in private between leaders.

Above all, there was never a question about a shared sense of values: democracy, liberty, freedom of speech, freedom of the press, human rights -- these were agreed norms. Everyone understood that allies stand together and solve their disagreements amicably and fairly. Every summit had a powerful, important and shared communiqué at the end -- demonstrating the cohesiveness of the alliance to the world. Therein lies the power of alliances, in which the sum of everyone’s contributions is greater than the simple sum of the parts.

There has never been a richer, more powerful or more important alliance than NATO in world history -- it represents around half of the world’s gross domestic product and boasts 3 million men and women (almost all volunteers) under arms.

More importantly, with 150,000 NATO troops under my strategic command on three continents, it never would have occurred to me that a national leader from any of the then-28 nations of the NATO Alliance would launch the sort of ad-hominem attack Trump directed at Trudeau at the G-7 summit. Greece and Turkey, for example, which have an unfortunate history of antagonism, are very careful to maintain polite decorum within NATO. To hear a U.S. president shatter that longstanding norm is deeply concerning, and will resonate throughout the world, much to America’s detriment.

When we next turn to NATO for assistance in a global hot spot -- from Africa to Afghanistan to our own shores -- the response may well be justifiably tepid. We should remember that the only time the NATO Alliance has executed an Article V commitment (an attack on one is an attack on all) was in the wake of the 9/11 attacks, when Europeans came to our defense, followed us into Afghanistan, and many of them also into Iraq.

Canada has been with the U.S. at every step of our national voyage for the last century, sending their sons and daughters into battle in World Wars I and II, the Korean War, Vietnam, the Cold War, Afghanistan and Libya. I remember visiting Canadian troops in Afghanistan, where they suffered grievous losses on the battlefield, and watching their jets fly with ours over Libya. There are plenty of worthy targets in the world for a U.S. president’s ire – Bashar al-Assad in Syria, Vladimir Putin in Russia, Nicolas Maduro, and of course Kim himself -- but Trudeau is not part of that set in any rationale sense.

What Trump risks with his constant attacks on NATO, his disrespect of European and Canadian leaders (including not only Trudeau, but most critically German Chancellor Angela Merkel), is the stability and longevity of the trans-Atlantic alliance. This is more than a “family spat,” as some administration officials have styled it. This division is beginning to feel permanent, at least in the context of the Trump administration.

Can we recover? Of course we can -- over time. But why risk the most important and battle-tested global alliance we have over a relatively minor disagreement on trade policy? We are vastly stronger with capable allies who believe in us, and Trump’s careless and thoughtless dismissal of an important NATO ally (echoed by his staff in the White House) will hurt us. All the more so when he lavishes praise on a truly brutal dictator in North Korea, who deserves to spend the rest of his life in a jail cell in the Hague under a conviction from the international criminal court.

Finding a diplomatic path forward on the Korean peninsula makes sense, and we should be pragmatists in finding ways to work with even a regime as odious as Kim’s dystopian nation. But we need to work from the inside out, which means shoring up the alliances that truly matter: NATO in general and Canada in particular. Breaking the trans-Atlantic bridge will distance the U.S. not just from allies, but from any claim to be the leader of the free world.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.