While President Trump is focused on Iran, a propitious moment to be the peacemaker in the Russia-Ukraine war could be slipping away. President Vladimir Putin of Russia and President Volodymyr Zelensky of Ukraine have been clear in recent months that they want the United States to re-engage and launch intensive negotiations to bring the fighting to an end.

They will not wait forever. Mr. Putin continues to speak approvingly of Mr. Trump, but some senior officials, as well as high-profile commentators, are highlighting U.S. assistance to Ukraine’s war effort and questioning whether the U.S. president can act as an effective and trusted intermediary. The Ukrainians continue to work closely with the Europeans on negotiating tactics. Some in Kyiv would like to see them more directly involved, which would widen the divide with Moscow, diminishing the chances for peace.

The argument for immediate U.S. re-engagement is compelling for two reasons.

First, the conflict has entered a dangerous escalatory spiral. Ukraine is stepping up its strikes against targets ever deeper in Russia, including energy infrastructure and military targets, with great effect. It is slowly cutting off the Crimean peninsula from the Russian mainland. For the first time, Russians far from the front are feeling the direct effects of war. The Kremlin is urging calm, but it is clearly worried that Kyiv’s strikes could demoralize Russian society, as it promises ever harsher retaliation against what it decries as terrorist attacks.

And retaliating it is. Russia is rapidly intensifying its aerial assault on Ukrainian cities and vital infrastructure, exploiting to devastating effect Ukraine’s dwindling stock of antimissile interceptors. The destruction is spreading and civilian deaths are mounting, as Ukraine scrambles to prepare for what it fears will be another bitterly cold winter.

At the same time, fears of a widening war are proliferating in Europe, as Ukrainian drones and Russian missiles — deliberately or not — violate NATO allies’ airspace and the Kremlin threatens action against Ukraine’s European backers. Moscow condemns what it views as Europe’s militarization and preparations for war, while European capitals argue that their actions are a necessary response to Russia’s aggression and threats to European security.

The situation is not directly analogous to the summer of 1914, but Barbara Tuchman’s “The Guns of August” stands as a cautionary tale of descent into an avoidable but ultimately catastrophic global conflict.

Second, and perhaps counterintuitively, the war is ripe for resolution. Both sides need to end the conflict as soon as possible. Kyiv is running out of fighters and is finding it increasingly difficult to mobilize men for the front lines. The longer the war continues, the less likely the millions of Ukrainians who have fled abroad will return. Continued conflict will only further raise the costs of reconstruction, which already run to the hundreds of billions of dollars.

Russia is facing its own demographic crisis and mounting difficulties in manning the front lines. The economic costs are rapidly rising and becoming more apparent as the economy stalls. The failure to invest in cutting-edge technologies such as artificial intelligence means Russia is falling farther behind the United States and China, the two countries against which it measures its worth as a great power.

Each day the war continues, the future looks grimmer. If it stopped today, however, each side could claim victory. Mr. Zelensky could rightly claim to have preserved what is most precious to Ukrainians: their independence, sovereignty, and European ambitions. Mr. Putin could extol Russia’s success in thwarting the West’s effort to inflict a strategic defeat on Russia and in “liberating” millions of Russians in Eastern Ukraine from what he portrays as a neo-Nazi regime in Kyiv.

Both sides may believe that continued fighting can improve their bargaining position. But the costs of war are steadily narrowing the political space for pursuing maximal objectives and increasing the value of a negotiated outcome.

In these circumstances, intensive negotiations are urgently needed to seize the moment and end the conflict. Only Washington has the combination of access, leverage and political weight to lead a comprehensive diplomatic effort. It alone can talk to both Moscow and Kyiv, as well as key European capitals, as appropriate. Ukraine and Russia cannot do this on their own — that is why they want Washington to re-engage. Russia will not accept any European nation as a serious interlocutor, given what it sees as Europe’s deep-seated anti-Russian animosity.

To be sure, close coordination with European governments will be essential — their military support, economic resources, sanctions policies and long-term commitment to Ukrainian independence are critical to achieving a durable settlement. But Washington should lead the diplomatic effort.

Mr. Trump should send his two envoys, Steve Witkoff and Jared Kushner, to Kyiv immediately to visibly re-engage the Ukrainians and then to Moscow for discussions with Mr. Putin. No one should expect a breakthrough; the task is to lay the groundwork for an intense period of shuttle diplomacy to bridge the yawning gaps between the two sides. Given Mr. Witkoff and Mr. Kushner’s other responsibilities, Mr. Trump should bolster their efforts with additional experts to spur negotiations.

Success is not certain, and it will not come as quickly as Mr. Trump would like. But the opportunity is real, and it may not last. If Washington is to test whether this war can be brought to an end, it should act now, before escalation closes the diplomatic opening that exhaustion has begun to create.