WASHINGTON (NewsNation) — President Joe Biden is traveling to Europe Wednesday to huddle with key allies as leaders try to prevent Russia’s war on Ukraine from spiraling into an even greater catastrophe.

Biden embarks 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.

Jake Sullivan, Biden’s national security adviser, said the president would coordinate with allies on military assistance for Ukraine and new sanctions on Russia. He added that Biden is working on long-term efforts to boost defenses in Eastern Europe, where more countries fear Russian aggression. The president is also aiming to reduce the continent’s reliance on Russian energy.

“This war will not end easily or rapidly,” Sullivan told reporters at a White House briefing on Tuesday. “For the past few months, the West has been united. The president is traveling to Europe to make sure we stay united.”

Sullivan said Vladimir Putin’s references to nuclear weapons at the beginning of the conflict are “something that we do have to be concerned about,” adding that Biden would be talking with allies about “potential responses” if the Russian leader takes that step.

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.

Biden will travel to Brussels where an emergency summit will be held. Biden will also participate in meetings of the European Union and the Group of Seven, which includes the world’s richest democracies.

He’ll then travel to Warsaw on Friday to meet Polish officials to discuss the enormous humanitarian strain caused by the Ukrainian refugee crisis. Biden is scheduled to meet with Polish President Andrzej Duda on Saturday.