The unnecessary, unjustified and unlawful war that followed convulsed the region, battered the global economy and exasperated the American public. And yet it may bequeath an accidental gift: a lasting aversion to military conflict with Iran and a chance to replace decades of failed policy with serious diplomacy.

The first factor is Mr. Trump himself. Barack Obama, who sought to engage Iran from the outset of his presidency, faced constant opposition from Republicans and many Democrats, wounding his nuclear deal. Mr. Trump tore it up three years later, and he has now become the instigator and loudest champion of a brutal war, the father of its failure and, one hopes, the bringer of peace. Think Nixon going to China — if Nixon had bombed China first.

No president has ever had more latitude to come to terms with the Islamic Republic, should he so choose. The administration’s deal has been mocked but not truly contested by the left, which wants the fighting to end. Washington’s Iran hawks scream, but having alienated Democrats long ago, they risk winding up politically homeless if they fully break with Mr. Trump. Nor can they easily separate themselves from the president. They applauded him for wrecking the Obama nuclear agreement and thrilled at his turn toward war just months ago. At this point, they can only hope that Mr. Trump fails to reach a broader nuclear accord so they can pressure him to return to war with even grander objectives.

They may succeed, of course. Mr. Trump could not be more erratic; translating his ambiguous “memorandum of understanding” with Iran into a detailed agreement poses a daunting challenge; and, in violation of his own deal, he almost immediately, and has since persistently, threatened to resume bombing should Iran’s behavior displease him. Still, for Mr. Trump, the use of enormous force against Iran is no longer a tantalizing option about which he can fantasize, but a bruising experience from which he had to retreat. The war he imagined delivered swift glory. The war he got was born unpopular and is dying an orphan.

If even a tenuous and hostile truce holds through the remainder of Mr. Trump’s term, then his successors, too, will know the price of war. They will have seen that the United States rapidly depleted enormous quantities of high-end munitions, needed in Europe and Asia, while failing to eliminate Iran’s missiles and drones. That Iran swiftly took control of the Strait of Hormuz, inflicting unmistakable costs on ordinary Americans, while the United States lacked the military options to force the strait open. That after all the bombs fell, the future of Tehran’s nuclear program still could be addressed only through negotiations, with a regime that emerged emboldened, vindicated and victorious. Future presidents will remember, in short, that the war was counterproductive on its own terms and came at the expense of every other foreign and domestic priority.