President Joe Biden heads to Europe this week as the leader of a coalition of western governments trying to stave off the worst military crisis in Europe since World War II. His mission: to keep it all from falling apart.

The alliance has largely marched in lockstep to confront Moscow — a remarkable demonstration of collective resolve not seen for many decades. But its bonds could soon be faced with deep strains, chief among them a creeping sense of powerlessness among the allies and reluctant admissions that the war may only end when Vladimir Putin decides it does.

“The nations of the free world are more united and more deterred and more purposed than at any point in recent memory,” said White House National Security Adviser Jake Sullivan on Tuesday, the eve of Biden’s trip. But he made clear that the path ahead would not be easy.

“There will be hard days ahead in Ukraine, hardest for the Ukrainian troops on the frontlines, and the civilians under Russian bombardment,” Sullivan added. “This war will not end easily or rapidly.”

The pressures Biden faces at home are mirrored among his European counterparts. While their domestic situations differ, all of them are trying to balance the moral imperative to act on behalf of Ukraine, the fears of further escalating the conflict and the economic costs of implementing an aggressive sanctions regime against Russia. The trickiness of Biden’s job, however, is felt more acutely given the expectations much of the world is now placing on his shoulders.

“There is a high degree of coordination and an extraordinary degree of success in producing a roster of quite stark sanctions,” said Ian Lesser, vice president of the German Marshall Fund and an expert on NATO. “But that brings its own challenge, which is to sustain that momentum through what is likely to be a long, protracted period of confrontation with Russia.”

Biden will partake in a trio of diplomatic summits in Brussels on Thursday to tighten those alliances before heading to Poland, the border nation which has received the largest influx of Ukrainian refugees in what has become one of the greatest humanitarian challenges in decades.

Presidents rarely travel overseas empty-handed but, to this point, the administration has been cagey as to what more deliverables Biden can or will provide. Officials said additional sanctions against Russia were expected along with a joint effort to crack down on Russia’s ability to evade those economic measures. There also is a promise of more funding and military equipment for Kyiv. But there are limits as to what Biden can do and how far he’s willing to go.

He has steadfastly opposed honoring Ukrainian President Volodymyr Zelenskyy’s wish for a no-fly zone, believing that trying to enforce one over Ukraine’s skies could lead to a catastrophic confrontation with Russia. While Polish leaders have proposed a peacekeeping mission in Ukraine, acting in part out of concerns for their own safety, such a move has little support within the Biden administration and among other allies, who view it as an escalation.

Once Putin gave the order to invade, the Biden team spearheaded a crippling array of sanctions from the West on Russia. Though some nations, namely Germany, needed more prodding, the White House aligned most of the allies to cut financial ties with Moscow. It was a significant ask of Europe, which has economies far more dependent on Russia than that of the United States.

And there is some fear that it could prove too difficult.

The biggest flashpoint could be on energy. To this point, much of Europe has not stopped importing gas and oil from Russia; doing so could cause an energy crisis on the continent and send prices skyrocketing around the globe. Zelenskyy, in emotional video messages to governing legislatures across the globe, has zeroed in on the existing energy ties as funding the Russian war machine. For Biden and Europe, the fear is that the alliance against Russia could fray as those energy demands hit harder.

Biden administration officials are hopeful that this week Europe may take some tentative steps away from Russia’s energy supply. But questions remain about what the U.S. could do to help European nations do so and whether that’s via American natural gas exports or agreements that could be brokered elsewhere.

Sullivan said Tuesday that Biden was aiming to “announce joint action on enhancing European energy security and reducing Europe’s dependence on Russian gas,” but did not provide further details.

Many European nations have sent weapons and equipment to Ukraine but the fate of MiGs fighter jets from Poland has become another flashpoint that threatens to cause friction among the allies. Where there is more uniformity among the NATO alliance is in the commitment to not send troops into Ukraine for fear of sparking World War III against a nuclear power.

But the biggest variable in holding the alliance together is time. While the sanctions from the West have placed Russia’s economy in a vice, Putin does not face the same domestic political considerations as others. Already punishing dissent in his country, he has the ability to withstand the economic doldrums — at least for a while — and continue his grueling advances despite the heavy losses in men and equipment.

At the moment, analysts said, the Russian forces have settled into a campaign of a brutal long-range bombardment of Ukrainian cities, targeting civilians and forcing them to flee. The West has little ability to stop them save for continuing to supply resistance fighters with weapons. The fear is that a bloody standoff that’s produced a humanitarian crisis turns into a prolonged stalemate. Or, that it could get worse. Among Western military officials, one foreboding question remains unclear: what to do if Putin unleashes a nuclear or biological weapon or launches a strike inside NATO territory?

“There’s still concern among some NATO allies when the Biden administration says ‘we will defend every inch of NATO territory,’ what exactly that might mean? I think they need to discuss that a bit in more detail,” said Daniel Hamilton, who served in a variety of senior positions in the State Department, including deputy assistant secretary for European Affairs.

“So much of it is political messaging, but that’s important, right now, in terms of solidarity,” Hamilton said.

The other complicating factor is China, which the White House fears could become Russia’s economic lifeline. United by a common hostility toward the West, Putin and Chinese President Xi Jinping had pledged deep ties between their nations. Biden warned Xi last week of “consequences” if he were to help Putin. And on Tuesday, Sullivan stressed that the U.S. has not seen the supplying of military equipment by China to Russia since Biden’s conversation with Xi, though officials have been careful not to make predictions about what might happen.

Added Sullivan: “The president made clear to President Xi the implications and consequences of any such provision of equipment and they very well understand.”

The partnerships Biden is working to keep intact were ones he started building within his first weeks as president, when he reached out to steady the United States’ longtime allies who were rattled by four tumultuous years of Donald Trump. His first trip to Europe, last June, was all but headlined “America’s Back” and featured Biden resolving to stand with the longtime partners, including the NATO alliance that Trump nearly torpedoed in years prior.

“He is seen as a very welcome departure from his predecessor when it comes to the value placed on transatlantic cooperation. And that’s very reassuring to Europeans,” Lesser said. “But it also sets up certain tests on the European side, where European leaders are looking to have their positive perceptions of the Biden administration and its approach reinforced.”

The outreach to allies was just part of Biden’s rebuke to Trump’s transactional, inward-looking “America First” foreign policy. He also declared that the next century would be defined by the ability of democracies to fend off rising authoritarianism around the globe. Driving home the point, Biden made a sharp break with Trump’s deference to Putin. During their summit last summer in Geneva, he explicitly warned the Russian leader to cease interfering with elections and condoning cyberattacks.

Putin didn’t listen.

Biden’s work with allies has been tested by the perception that Russia has not responded to his demands. It’s also been strained by his handling of the chaotic early days of the military withdrawal from Afghanistan last summer. Hoping to calm concerns, the administration involved allies in the early stages of the current crisis. The White House, along with the United Kingdom and other nations, released intelligence in nearly real-time as it tried to ward off Putin from invading and prepare their populaces for the possibility of war.

“There’s a great deal of transparency about what’s actually happening on the ground,” Lesser said. “But beyond that, it becomes very difficult to judge exactly how Putin is going to view the endgame in Ukraine, what he’s willing to settle for [and] what can deter him. This is precisely the kind of conversation that the president is going to have to have in Europe because everybody’s facing essentially the same dilemmas.”