Barack Obama spent years trying to coax the Europeans into doing more for their collective defense. Mr. Trump, by presenting Europe with a powerful external threat — him — could force them to finally follow through.

The potential downsides are abstract but significant. No one is sure how many times Mr. Trump can shake the foundations of the international order before it collapses, either in part or in whole. And no one can say what will happen if it does.

But we do know, at least, what the current order supports. And Americans know this particularly well because it was the United States that built this system, in large part to serve its own interests.

NATO may be the clearest case. Though Mr. Trump has characterized it as a kind of American protection service for freeloading Europeans, it was intended to keep the Europeans unified and yoked to American leadership. Not only would they never again threaten the United States, their reliance on American power enlists them on its behalf.

Mr. Trump has boasted of increasing American military spending to $623 billion this year, from $603 billion in 2016. But American collective defense with NATO effectively adds $312 billion — the combined defense budgets of fellow members — in military power. That power applies to actual wars as well as the less visible but consequential projection of unused firepower, whether in deterring Russian or Chinese ambitions or keeping seafaring lanes clear.

But NATO, like other aspects of the American-led order, consists of more than its collective hardware. It is held together by trust: that its members will come together in mutual support, and not only when faced with a common threat like Russia. That trust, going against the self-interested instincts of the individual countries, is radically new in the world. Its resilience is unknown.