What if the United States effectively ceases to be part of the Western world?

That question, previously a far-fetched idea from the more abstruse reaches of political theory, has suddenly appeared on the lips of Europe's most prominent leaders. It is the realization of a European nightmare – a future without the United States – that could also, in important ways, turn out to be the best thing to happen to the continent and to the wider West.

Donald Trump's calamitous tour through Europe last week saw the U.S. President pointedly savage or brusquely rebuff virtually all the political, trade, military and ecological alliances that form the core of the postwar European and North American peace; in the days after the tour, he went even further, attacking allies and cancelling the world's most difficult and important international agreement, the Paris climate accord.

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In doing so, he confirmed the worst fears of many: that the leadership of the United States no longer appears to be adhering to core Western values and principles.

German Chancellor Angela Merkel expressed this fear rather bluntly in a Munich beer tent on Sunday, to a sustained ovation from her audience of Bavarian conservatives. She began in visible anger: "The times in which we could completely rely on others are somewhat over – that is what I experienced in the last few days," she said of her meetings with Mr. Trump.

Yet, she continued with a defiant message, seemingly aimed at the rest of the Western world: "We Europeans must really take our fate into our own hands … we alone have to fight for our own future, as Europeans, for our destiny."

Those words were echoed, as the week unfolded, with no less anger and defiance, by other prominent European leaders, including French President Emmanuel Macron and European Commission President Jean-Claude Juncker.

The shared set of values – broadly if imperfectly in place since the end of the Second World War, which have more or less united North America, most of Europe, Australasia and the democratic parts of Asia – have all been threatened this week. The U.S. and Europe (as well as Canada) have drifted apart before – for example, during the 2003 invasion of Iraq – but those deep differences were always within the framework of institutions and broad understandings that have underpinned the West. Those same institutions have now all come under attack, from the leader of their wealthiest country.

When we speak of the Western world today, we're talking about a set of values and principles and commitments to institutions more than about any geographic location. What grew out of the surviving democracies of Western Europe and North America after the Second World War has expanded to include scores of nations, on almost every continent, committed to broad principles of democracy, justice, human equality and international co-operation, around shared projects of prosperity, sustainability and security.

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What Mr. Trump did this week – and what he has suggested he's been wanting to do, throughout his presidency – is shift the United States into a pointedly anti-Western stance.

He has threatened the military union, by refusing to endorse the mutual-defence pact in NATO's Article 5, and by refusing, as well, to condemn Russia's military incursions; he has threatened the economic and trade union, by blasting Germany's exports, promising trade barriers, and exiting or challenging crucial trade agreements; he has threatened the hard-fought efforts to save the world from calamity, by withdrawing his country entirely from the Paris climate treaty on Thursday, leaving the United States in an anti-Western, isolationist alliance whose only other members are Syria and Nicaragua.

He then used that announcement to lash out, in language borrowed from the extreme fringes of politics, at the very idea of international agreements, shared climate solutions and democratic alliances.

Most disturbingly, he has turned against most of his country's most trusted democratic allies, ignoring their entreaties and sometimes lashing out against them, while allying in warm public embrace and uncritical agreement with the autocratic leaders of the Philippines, Saudi Arabia, Turkey and Russia. This, more than anything, has signalled his apparent retreat from the West.

The Western alliance has already lost Russia to authoritarianism, and Turkey has all but fallen off the ledger. Britain's turn to extreme anti-Europe politics is not really comparable: While potentially devastating to the U.K.'s own economy, it is a self-inflicted wound that does not affect the country's role in NATO or on climate issues, international aid and broader trade alliances, or significantly alter the postwar order.

Mr. Trump's turn to the dark side is something else entirely, threatening to turn the G7 into a G6, and the unprecedented level of international co-operation seen during the past decade into a historic moment of frozen progress and dark distrust.

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And yet, it could also spur the rest of the democratic world into a new start.

Europe has been enjoying a minor renaissance this year, with a powerful economic recovery and fast-falling unemployment in France, Germany and the Netherlands, and astonishingly successful recoveries in Spain, Portugal and Ireland. The euro has stabilized, the post-2008 crisis has finally ended, the political union is more cohesive and co-operative than it has been this century, and optimistic voters have generally turned away from extremes and toward pragmatic politics. Likewise, Canada, Australia, and especially Japan are enjoying buoyant moments economically and politically: This should be a heady moment in the Western world.

The shock of Mr. Trump's anti-Western thrust appears to have reminded many jaded Europeans why their political and economic union exists: It is the only alternative to the isolationism that destroyed the continent's peace in the last century.

Suddenly, the ever-chaotic and overbureaucratized tedium of the European Union, the sad compromise of the Paris agreement, and the ineffective mess of NATO look so much better than the Hobbesian alternative embodied by Mr. Trump. For citizens of other Western countries, including Canada, the frustrating experience of trade agreements, United Nations bodies and climate pacts suddenly seems worth it. We are reminded of the sense of threat and menace that led us to create these difficult international solutions in the 1940s and 1950s.

This week that new spirit was already visible. Ms. Merkel and Mr. Macron began to speak of a "new push" in European relations, and declared that their countries would draft a road map toward a more independent Europe. Sigmar Gabriel, Germany's foreign minister (and vice-chancellor) has outlined a plan that would build up political co-operation within the euro-zone countries, launch a joint German-French defence fund and deepen political integration.

Cecilia Malmström, the Swedish politician who serves as Europe's Trade Commissioner, described this new sensibility in an interview with the German magazine Der Spiegel: "Trump's election has opened many people's eyes in Europe to how valuable the European Union and international co-operation are," she said. "If the U.S. turns away from the international system, then we will simply have to look after it ourselves."

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For too long, Westerners have relied on U.S. might and wealth as their crutch, their backstop, as the foil that allows them to put off larger commitments to one another. By walking away from the values that underpin the postwar democratic world – temporarily, we can hope – the United States may have created an emergency that has inadvertently brought the rest of the Western world closer, to overcome this gigantic hole in our collective centre. As Canadians, and members of that threatened community, we owe it to our neighbours to help fill that hole.

Doug Saunders is The Globe and Mail's international-affairs columnist.