President Joe Biden is set to consult with allies from NATO’s eastern flank in Poland on Tuesday as the Russian invasion of Ukraine edges toward an even more complicated stage.

After paying an unannounced visit to Kyiv, Biden made his way to Warsaw on Monday on a mission to solidify Western unity as both Ukraine and Russia prepare to launch spring offensives. The conflict — the most significant war in Europe since World War II — has already left tens of thousands dead, devastated Ukraine’s infrastructure system and damaged the global economy.

“I thought it was critical that there not be any doubt, none whatsoever, about U.S. support for Ukraine in the war,” Biden said as he stood with Ukrainian President Volodomyr Zelenskyy in Kyiv before departing for Poland. “The Ukrainian people have stepped up in a way that few people ever have in the past.”

Biden is scheduled to meet with Polish President Andrzej Duda and deliver an address from the gardens of Warsaw’s Royal Castle on Tuesday, where he’s expected to highlight the commitment of the central European country and other allies to Ukraine over the past year. On Wednesday, he’ll consult with Duda and other leaders of the Bucharest Nine, a group of the easternmost members of NATO military alliance.

White House national security adviser Jake Sullivan said Biden would underscore in his Warsaw address that Russian President Vladimir Putin wrongly surmised “that Ukraine would cower and that the West would be divided” when he launched his invasion.

“He got the opposite of that across the board,” Sullivan said.

While Biden is looking to use his whirlwind trip to Europe as a moment of affirmation for Ukraine and allies, the White House has also emphasized that there is no clear endgame to the war in the near term and the situation on the ground has become increasingly complex.

The administration on Sunday revealed it has new intelligence suggesting that China, which has remained on the sidelines of the conflict, is now considering sending Moscow lethal aid. Secretary of State Antony Blinken said it could become a “serious problem” if Beijing follows through.

Biden and Zelenskyy discussed capabilities that Ukraine needs “to be able to succeed on the battlefield” in the months ahead, Sullivan said. Zelenskyy has been pushing the U.S. and European allies to provide fighter jets and long-range missile systems known as ATACMS — which Biden has declined to provide so far. Sullivan declined to comment on whether there was any movement on the matter during the leaders’ talk.

With no end in sight for the war, the anniversary is a critical moment for Biden to try to bolster European unity and reiterate that Putin’s invasion was a frontal attack on the post-World War II international order. The White House hopes the president’s visit to Kyiv and Warsaw will help bolster American and global resolve.

“It is going to be a long war,” said Michal Baranowski, managing director of the German Marshall Fund East. “If we don’t have the political leadership and if we don’t explain to our societies why this war is critical for their security ... then Ukraine would be in trouble.”

In the U.S., a poll published last week by The Associated Press-NORC Center for Public Affairs Research shows that support for providing Ukraine with weapons and direct economic assistance is softening. And earlier this month, 11 House Republicans introduced what they called the “Ukraine fatigue” resolution urging Biden to end military and financial aid to Ukraine, while pushing Ukraine and Russia to come to a peace agreement.

Biden dismissed the notion of waning American support during his visit to Kyiv.