That clanking sound you hear is the United States pulling up the drawbridge, trying to wall itself off from the rest of the world.

Donald Trump’s ringing cry in his inaugural address to put “only America first, America first” (he repeated the phrase for emphasis) is the most inward-looking vision that any incoming American president has put forward in many decades.

Coupled with his unrelenting focus on fixing what he labelled the “carnage” afflicting the American economic heartland (“rusted-out factories scattered like tombstones across the landscape of our nation”), it sends a strong signal that the United States intends to pull back from its commitments around the globe and from its traditional leadership of what used to be known as the “free world.”

Many of those who have criticized U.S. dominance, even hegemony, in far-flung parts of the world, may well welcome this retreat. It has too often led to bloody semi-colonial wars, including in Vietnam and Iraq, as well as interference in many countries.

But be careful what you wish for. Since the end of the Second World War the United States has acted as the indispensable linchpin and backstop for the international order, including key institutions such as the United Nations, NATO and the global financial structures.

Allies like Canada often chafed against U.S. dominance, but realized there was no real substitute for the leadership of Washington. Every time a crisis broke out, from the Balkans to the Middle East, American allies waited to take their lead from the U.S. president.

No one should be surprised that, for all Trump’s loud talk, he wants to pull back from foreign entanglements. He foreshadowed the impending U.S. retreat all through his campaign, and he hammered away at the same theme earlier this week when he dismissed NATO as obsolete, predicted the European Union would fall apart, and basically wrote off the importance of the U.S.-European relationship. All the while continuing to make nice with Russia’s Vladimir Putin.

But while it shouldn’t come as a shock, it’s hard to overstate what a tectonic shift all this amounts to in global relations. The new president spent much of his inaugural speech decrying how “we’ve made other countries rich, while the wealth, strength and confidence of our country has dissipated over the horizon.”

And further: “For many decades, we’ve enriched foreign industry at the expense of American industry; subsidized the armies of other countries, while allowing for the very sad depletion of our military. We’ve defended other nations’ borders while refusing to defend our own.”

American presidents since Franklin Roosevelt have accepted that the United States has a special leadership role, and the cost it bears has more than paid off in world-wide economic strength and unparalleled technological and financial dominance – not to mention Washington’s ability to shape international institutions to suit its own interests. The United States did not become a superpower by shrinking from the world.

Now along comes Trump with the most isolationist message from Washington since the 1930s, a time when all the major powers huddled behind their own borders, international trade collapsed and the world suffered through the Depression and the rise of the dictators. Americans (quite understandably) were preoccupied with their own domestic woes and had no interest in meddling in other peoples’ problems.

But the catastrophe of world war and the Cold War that followed changed all that. Through seven decades and a dozen presidents of both parties, the United States accepted the burden of world leadership – and the abundant rewards that came along with it. It led the drive to open borders and build a world order with deeply integrated economies.

Previous presidents, from John F. Kennedy to Ronald Reagan, emphasized what they saw as the duty of the United States to uphold freedom around the world – not just tend the economic garden at home.

For Donald Trump, however, the world beyond the borders of the United States appears to hold only threats, not promise. His speech included a pro forma mention of the need to “reinforce old alliances and form new ones,” but it’s clear where his heart lies. It’s America first.

Of course, it’s inevitable that the world should rebalance itself over time. The post-Second World War order was based on resisting communism and came into being at a time when the rest of the world either lay in ruins or was still in the throes of colonialism.

But if the U.S. pulls back, it’s not clear how that will affect other countries, especially at a time when vital parts of the world, especially in the Mideast, are still in turmoil.

Trump himself promised to “unite the civilized world against radical Islamic terrorism” – but doesn’t seem to appreciate how that would involve getting drawn back into expensive conflicts abroad. Walling America off from the world will surely prove a lot more difficult than Trump would like.

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It’s also not clear who might fill the void left by a retreating Washington. Some other countries, including China and Russia, are clearly eager to act in what they regard as their spheres of interest. But that will bring its own share of problems.

As the United States enters an era of unabashed “America first” populism, Canada and its other allies won’t just feel the sting of protectionism on economic matters. They will also have to figure out who will be their ultimate leader when things go wrong.

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