President Joe Biden on Tuesday warned of “hard and bitter days ahead” as Russia’s invasion of Ukraine nears the one-year mark, but vowed that no matter what, the United States and allies “will not waver” in supporting the Ukrainians.

A day after his surprise visit to Kyiv, Biden used a strongly worded address in neighboring Poland to praise allies in Europe for stepping up over the past year and to send a clear message to Russian President Vladimir Putin that “NATO will not be divided, and we will not tire.”

“One year ago, the world was bracing for the fall of Kyiv,” Biden said before a crowd of thousands outside Warsaw’s Royal Castle. “I can report: Kyiv stands strong. Kyiv stands proud. It stands tall and, most important, it stands free.”

With Russia and Ukraine each preparing spring offensives, Biden insisted there will be no backing down from what he’s portrayed as a global struggle between democracy and autocracy — though polling suggests American support for ongoing military assistance appears to be softening.

“Democracies of the world will stand guard over freedom today, tomorrow and forever,” Biden declared. The U.S. and allies will “have Ukraine’s back.”

Biden’s speech came a day after his unannounced trip to Kyiv, where he met with Ukrainian President Volodymyr Zelenskyy and walked the city streets as an air raid siren blared.

Last year, weeks after Russian forces began their attacks on Ukraine, Biden delivered a harsh condemnation of Putin from the gardens of the castle. Speaking Tuesday to a crowd that included Polish citizens and Ukrainian refugees — and millions more following in Ukraine, Russia and around the world — he made his case that Putin’s war has been a failure.

“When President Putin ordered his tanks to roll into Ukraine, he thought we would roll over. He was wrong,” Biden said.

The president also declared “the democracies of the world have grown stronger” while the world’s autocrats — including Putin — have become weaker.

“Autocrats only understand one word — no, no, no,” Biden said. “No, you will not take my country. No, you will not take my freedom. No, you will not take my future.”

Earlier in the day, Biden met with Polish President Andrzej Duda as he began consultations with allies to prepare for an ever-more-complicated stage of the war.

“We have to have security in Europe,” Biden said at the presidential palace. “It’s that basic, that simple, that consequential.”

He described NATO as “maybe the most consequential alliance in history,” and he said it’s “stronger than it’s ever been” despite the Russian leader’s hopes that it would fracture over the war in Ukraine.

Duda praised the American president’s visit to Kyiv as “spectacular,” saying it “boosted morale of Ukraine’s defenders.”