And if others do more, they may not be the right others, and what they do may not serve America’s interests. Russia has been doing more in Europe since the United States drew down, and more in Syria as a result of America’s doing so little.

China is doing more in the South and East China Seas, and its activities will greatly complicate American military operations in defense of allies, in preserving the free flow of commerce and even in protecting its own territory.

The Islamic State in part grew out of the United States doing too little to consolidate the gains of the surge in Iraq and caring too little about the Syrian government’s depredations against its people. None of these are outcomes that advance American interests.

Now imagine the longer term. China is already demonstrating that not only will it not play by the rules of the American-led order, but it also intends to write and enforce new rules. Absent American opposition, it will continue to force smaller, weaker states in East Asia into submission and expand its control over sea traffic. It will use technology to monitor, restrain and penalize critics worldwide. A Chinese-led world order would be one of privileges rather than rights, power rather than law, tribute rather than alliance.

That’s a very costly peace — if it even succeeds at sustaining peace. More likely, if the United States does not sustain the order, a rising power will eventually force it to defend its interests or succumb. That is what happened in every power transition except the one between Britain and the United States — an exception born of their democratic similarities, and one unlikely to be repeated with the United States and China.

Mr. Trump’s attack on the liberal world order is not just about the price America pays for it. He seems bent on destroying the friendships and respect that bind America and its allies. If he succeeds, America will be seen as — and may even become — no different from Russia and China, and countries will have no reason to assist America’s efforts rather than theirs.

America has been dominant for so long that it takes for granted outcomes that support its policies and interests, and undervalues the systemic advantages of institutions and norms. Yet Mr. Trump may end up proving an illiberal preserver of the liberal international order. By calling into question so many fundamental elements of the system that the United States built in the devastated aftermath of World War II, he is forcing Americans to imagine a world in which the United States does not tend the garden of international order, as George Shultz describes foreign policy.