Biden left Europe in a jubilant mood, fresh off essentially welcoming Sweden into the alliance and displaying transatlantic unity that is deeply personal to the president. A day earlier, in the week’s centerpiece speech from the NATO summit in Lithuania, Biden defended his policies and rallied democracies to keep up the fight against tyranny in Ukraine and around the world. And he was able to help ease concerns from Ukrainian President Volodymyr Zelenskyy, who over the course of the Vilnius summit appeared to abandon his fury that NATO was slow-walking his country’s admission to the alliance.

But loose ends and persistent questions remain about the next steps in Europe’s largest land conflict since World War II. And the problem for Biden is that much of what comes next isn’t wholly in his control.

Congress could scuttle a F-16 warplane transfer to Turkey that could make Ankara think twice about green lighting Sweden’s accession. What’s more, those long-term commitments to Ukraine — which Biden likens to America’s support for Israel — will require congressional lawmakers to commit to staying the course for years. Ukraine’s counteroffensive has sputtered, potentially making allies and senators reconsider their support.

There are real worries about Ukraine’s arsenal, its reduced supply forcing the U.S. to make the controversial decision to send over cluster bombs. The whispers of doubt among Republicans back home have grown louder over continuing to fund Kyiv at current levels. On the sluggish start to the Ukrainian counteroffensive, Biden told reporters that even Zelenskyy acknowledged to him that it was “a hard slog.”

And then there’s next year’s U.S. election, where Trump, the Republican frontrunner, could likely dismantle what Biden helped build within months of returning to the Oval Office.

And indeed in Helsinki, the presence of two other men shadowed the day.

The echoes of Trump and Putin’s July 2018 summit reverberated throughout Biden’s meeting with Finnish President Sauli Niinistö. Thursday’s news conference was held in the exact same room in Finland’s presidential palace where Trump — when asked by a reporter, “who do you believe?” — made clear that he sided with Putin over the conclusions of his own government.

Now, Putin stands as an international pariah, a global outcast after launching a brutal invasion of Ukraine more than 500 days ago, as his military falters and much of the Western World rallies around Kyiv. The expansion of NATO includes the entry of Finland, which shares an 800-mile border with Russia and had to cast aside decades of neutrality to join the alliance.

Trump, meanwhile, has telegraphed that he would abandon Ukraine and revisit staying in NATO. Both Biden and Niinistö tried to downplay the possibility that a return of Trump would threaten the alliance.

Biden said he would “absolutely guarantee” that the U.S. would remain in NATO, touting support from both parties and downplaying the power of “the extreme elements of one party.”

But he later softened by saying that “no one can guarantee the future, but this is the best bet.”

If the administration offered carrots to allies, the congressional delegation to this week’s NATO summit wielded a stick in their private meetings. Whether it was heads of state or legislative counterparts, their message was that NATO members needed to boost their defense spending. The U.S. couldn’t do it all, they asserted.

“In the Congress, there is a growing, serious frustration with the lack of other countries keeping their commitments,” said Sen. Dan Sullivan (R-Alaska), a long-time advocate for getting NATO members to spend at least 2 percent of their GDP on their security. “It’s a topic that unifies Democrats and Republicans, because it’s an issue of fairness. We all want a sustainable, strong NATO, but it’s not sustainable and strong when there’s a sense that the burden has not been shared.”

Zelenskyy, meanwhile, left Vilnius disappointed that he didn’t score imminent NATO membership for Ukraine. But while that’ll be a political blow at home, he didn’t leave empty handed: He got billions of dollars in security and economic assistance pledges from NATO members and G-7 nations that are designed to give Kyiv an edge deep into the protracted war.

However, even the bipartisan congressional delegation expressed caution about admitting Ukraine into the club too quickly. Still, Sen. Dick Durbin (D-Ill.), the Senate’s No. 2 Democrat, said Zelenskyy should feel confident “an offer will be made to him at some point.”

In its public and private messaging, the Biden administration repeated that Ukraine still wasn’t ready for NATO membership, even if the war with Russia somehow ended soon. On Thursday, he reiterated that Ukraine’s future was in NATO but that he could not offer any sort of timetable.

Ward contributed reporting from Vilnius, Lithuania.