With Europe facing its most precarious future since World War II, President Joe Biden will huddle with key allies in Brussels and Warsaw this week as the leaders try to prevent Russia's war on Ukraine from spiraling into an even greater catastrophe.

Biden embarked Wednesday on a four-day trip that will test his ability to navigate the continent's worst crisis since WWII ended in 1945. There are fears that Russia could use chemical or nuclear weapons as its invasion becomes bogged down in the face of logistical problems and fierce Ukrainian resistance.

Humanitarian challenges are growing as well. Millions of refugees have fled the fighting, mostly by crossing the border into Poland, and the war has jeopardized Ukraine's wheat and barley harvests, raising the possibility of rising hunger in impoverished areas around the globe.

On Wednesday, Biden will attend a hastily arranged NATO emergency summit, a Group of Seven meeting and a European Council meeting. 

Aboard Air Force One on Wednesday, Jake Sullivan, Biden's national security adviser, said Biden will meet with NATO Secretary-General Jens Stoltenberg before the summit.

Among the subjects that will come up during the summit are continuing support for Ukraine and establishing a “longer term game plan for forces and capabilities are going to be required in those eastern flank countries,” Sullivan said. “So that we’ve got a long-term footprint that is matched to the new security reality that's been created both by Russia's full-scale invasion of Ukraine and by what's happening in Belarus,” where U.S. officials believe more Russian forces will gather.

At the G7 meeting, the United States will agree on an initiative to coordination on sanctions enforcement so that Russia cannot evade sanctions on their own or with the help of other countries, Sullivan said. The U.S. also will announce a new package of sanctions targeting political figures and oligarchs, he added.

Asked what more sanctions the Biden administration might have left to impose on Russia, Sullivan said the U.S. is constantly looking for ways to “tighten the screws.” 

“What we've done on the financial sanctions and the export controls is create a vise that is squeezing harder and harder with each passing day and week,” he said. “And then as we see the impact and we see efforts to evade, we think about adjustments we need to make to ensure that vise continues to tighten.”

At the G7 meeting, Biden also will discuss the “global energy picture and the global food security picture” related to the war.

Sullivan said it’s a “major priority” for the U.S. and its European allies to reduce their dependence on Russia oil and gas and said “we will have more to say on this subject specifically on Friday, particularly with … European Commission President (Ursula) von der Leyen.”

Biden and leaders also will discuss Russia’s suggestions that the potential use of nuclear weapons is not off the table.

“We haven't seen anything that's made us adjust our posture, our nuclear posture, but it's of course something that we will have to continue to stay in close consultation with allied partners on, as well as communicate directly to the Russians on,” Sullivan said.

Biden will travel to Warsaw, Poland, on Friday. On Saturday, he will meet with Polish President Andrzej Duda of Poland and later give a speech on efforts to support Ukraine in the war and its people. 

The president also will “engage with U.S. troops who are now helping to defend NATO territory,” White House deputy press secretary Karine Jean-Pierre said.

Duda, whose country suffered a brutal Nazi occupation during World War II, has compared Russian actions in Ukraine to Adolf Hitler's infamous SS forces. Visiting Bulgaria on Tuesday, Duda said Putin's army "is behaving in exactly the same way." He said he hoped that those responsible for attacks on civilians would be brought before international courts.

Polish leaders have pressed for a Western peacekeeping mission to intervene in Ukraine, a step that the U.S. and other Western allies worry could lead to a broadening of the war. The Polish leadership also wants an increased military presence along NATO's eastern flank.

Sullivan said Tuesday that Biden's trip to Poland is an important opportunity to "meet with a frontline and very vulnerable ally." 

Biden's visit to Poland follows on Vice President Kamala Harris' visit to Warsaw and Bucharest earlier this month. While Harris was in Poland, Duda called on the Biden administration to expedite visa procedures for Ukrainians who have family living in the United States so that they could resettle in the U.S. at least temporarily.

Sullivan's description of Biden's trip was another sign that the crisis is entering a new and uncertain phase.

After the initial invasion failed to topple Ukraine's government, the war has become a grinding endeavor for Putin, who is relying on airstrikes and artillery that are devastating civilian communities. Negotiations between Ukraine and Russia have not produced a cease-fire or a path to ending the conflict, and the U.S. continues to rush weapons like anti-tank missiles to Ukrainian forces.

The war's ripple effects are also spreading. Biden warned that Russia could be planning cyberattacks that would affect U.S. companies, and he spoke to Chinese President Xi Jinping on Friday to warn him against backing Russia with military or financial assistance. Meanwhile, a top State Department official visited India this week shortly after that country decided to purchase more Russian oil.

"This is one of those decisive moments for an American leader that defines their legacy internationally," said Timothy Naftali, a presidential historian at New York University.