LONDON — The world’s first-ever theory of war, concerning two Greek cities, might appeal to President Trump.
Athens and Sparta, old allies, went to war 2,500 years ago, according to the historian Thucydides, when plucky Athens grew powerful enough to rival Sparta, then the pre-eminent Greek power. The natural order could not abide two peers side by side, making conflict inevitable.
But that was the old world of ruthless self-interest and zero-sum competition. Today, nations have some idea of how to rise above those instincts.
Consider the half-century alliance between the United States and Europe. A unified Europe is America’s closest peer in economic and military strength. Under Thucydides’s theory, which held for millenniums, conflict between them should be inevitable — and yet it is unthinkable.
The scaffolding with which nations have lifted themselves above the old ways is sometimes called the liberal international order: alliances, free trade and organizations like the United Nations. It remains a work in progress, but one at least intended to hold all nations in peaceful coexistence.
Mr. Trump, in his instinct for the old ways, is pushing and pulling at that scaffolding, and has taken several opportunities to shake its foundations during his weeklong tour of Europe.
Distrustful of all agreements except those he forged himself, Mr. Trump treats even allies as competitors. His is the world of Athens and Sparta. And while that hardly means a descent into war, Mr. Trump’s instincts have brought conflict in other forms. He has launched sweeping trade wars and seems in perpetual diplomatic rows, particularly with allies.
Is the international order as we know it at risk from Mr. Trump, as establishment foreign policy voices in Europe and the United States increasingly warn? His performance in Brussels, where he threw a NATO gathering into chaos by issuing vague threats of going it alone (and then backing down), highlights the stakes of his approach.
There are potential gains. Though Mr. Trump has yet to show significant concessions from the Europeans, they may deliver some out of fear of losing American support, something only this president could credibly threaten.
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 another $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.
Every time Mr. Trump threatens or berates an ally or calls NATO obsolete, even if he does so to secure greater European commitments to upholding that order, he chips away at the trust holding it together.
That distrust filters down to populations — polls show that European approval of the United States has plummeted — and it steers leaders’ incentives away from helping the United States. A recent poll found that a plurality of Germans would favor an American military withdrawal from their country.
A full-scale collapse of the Western security order is so difficult to imagine that it is detailed only in fiction. Incremental breaks are more conceivable, such as a kernel of doubt in the United States’ pledge to defend tiny Eastern Bloc members like Estonia or Latvia as if they were American soil. Might Russia be tempted to test that commitment?
Maybe all this is just another aspect of the cost-benefit tactics that can be seen in Mr. Trump’s approach to other issues, particularly the one that appears to most captivate him: trade.
The system of global free trade has downsides — when an industry shifts from one country to another, masses of workers can pay the price — but it was set up as a kind of permanent win for the United States. Openness exploits the sheer size and development of the American economy so that its goods and services can dominate internationally. And it lowers the price of goods, raising the American quality of life.
When Mr. Trump shakes the system of global trade by imposing tariffs and issuing demands, perhaps he is try to coerce other nations into engineering that system to be even more favorable. The cost to the average American family is nominal, though cost to American producers could be higher.
Longer term, might this erode American dominance of the international trade system? China, hit heavily by Mr. Trump’s tariffs, is trying to position itself as the new responsible steward of global trade — a difficult sell, but with Chinese economic links in Europe and elsewhere growing, not unimaginable.
The stakes, largely hypothetical, are uncertain. But every disruption makes the United States a less attractive partner, source of investment or place to do business. International economic isolation can be self-perpetuating as businesses and currency reserves flee abroad. Even if there is no single moment of breakdown, self-imposed isolation, should it continue too long, could exceed the benefits of rejiggered trade relationships.
Could things ever really get that far? It is difficult to say for sure, in part because Mr. Trump’s own motivations can be hard to nail down. It is unclear how far he will go.
Some see, in Mr. Trump’s affinity for strongmen and skepticism of migration, an ideological through line that extends back to old nationalist ideas of clashing civilizations and going it alone. Thucydides still holds in this worldview — Stephen K. Bannon and other early advisers are said to be great fans. And skepticism of multilateralism runs deep in the grass-roots of the American right, even if official Republican Party orthodoxy tends to be more internationalist.
Others suspect Mr. Trump is driven less by lofty ideology than by moment-to-moment impulse. Maybe, rather than strategically bent on deconstructing the international order, he is temperamentally inclined toward actions that have that effect.
European officials often talk of appeasing his ego or containing what they see as his inclinations toward disruption for its own sake. They point out that Mr. Trump has consistently mischaracterized European defense spending as payments to the United States.
Perhaps Mr. Trump is simply importing the brinkmanship and bluffing he espoused, in his real estate days, as negotiating tactics. Hints that he might give up on NATO, in this view, are not sincere assaults on the alliance but mere cage rattling meant to spook the Europeans into complying with decades-old demands to spend more on defense.
Still, if that is the strategy, its gains can be obscure. In his telling, Mr. Trump provoked an emergency meeting at this week’s NATO gathering to pressure European members to “substantially up their commitment” to defense spending. It worked, he said: “They’re going to up it at levels that they’ve never thought of before.”
Minutes later, Emmanuel Macron, the French president, said that the Europeans had merely reaffirmed previously agreed-upon spending plans.
It was not lost on participants that Mr. Trump had claimed to resolve a crisis he himself had provoked. Nor that his impromptu news conference, where he spoke of extracting concessions, coincided with morning news programs back in the United States.
The global order does bring downsides for the United States, which is constrained by its role as the reliable keystone at the center of it all. It’s thankless and often frustrating work.
Other countries are free to act as spoilers, threatening the greater system to force some concessions, knowing the Americans will keep that system together. This role, at least traditionally, has not been considered available to the United States; you cannot be both leader and spoiler.
But Mr. Trump is testing that assumption in ways no president has before. If he is on to something, the gains will come. If he’s not, some fear that the costs could be permanent.