WARSAW, Poland — Twenty-five years ago, Joe Biden visited Warsaw, Poland, with a warning: Even though the Soviet Union had collapsed, some of NATO’s original members weren’t doing enough to ensure the alliance’s collective defense.

“Now it is time for the people of Western Europe to invest in the security of their continent for the next century,” said Biden, then a U.S. senator.

Biden, now president, speaks again here Saturday as European security faces its most precarious test since World War II. The bloody war in Ukraine has entered its second month, and Western leaders have spent the week consulting over contingency plans in case the conflict mutates or spreads. The invasion has shaken NATO out of any complacency it might have felt and cast a dark shadow over the continent.

U.S. national security adviser Jake Sullivan said the speech will outline the “urgency of the challenge that lies ahead” and “what the conflict in Ukraine means for the world, and why it is so important that the free world stay in unity and resolve in the face of Russian aggression.”

Biden’s remarks will end a four-day trip that included an earlier stop for a series of summits in Brussels. While in Warsaw, he dropped in Saturday morning on a meeting between U.S. and Ukrainian foreign policy and defense leaders. He also planned to visit with Polish President Andrzej Duda and meet with Ukrainian refugees and the aid workers who have been helping them.

The president’s hotel is across the street from Warsaw’s central train station, a major arrival point for Ukrainian refugees. Some 3.5 million have fled the country; 2 million are in Poland. Aid workers in colorful vests circulated through the terminal, and signs pointed to tents outside where refugees could eat for free.

Biden previewed his closing speech during appearances Friday in Rzeszow.

“You’re in the midst of a fight between democracies and oligarchs,” the president told members of the U.S. Army’s 82nd Airborne Division as he visited their temporary headquarters. “Is democracy going to prevail and the values we share, or are autocracies going to prevail?”

During a later briefing on the refugee response, Biden said “the single most important thing that we can do from the outset” to force Russian President Vladimir Putin to stop the war “is keep the democracies united in our opposition.”

Biden praised the humanitarian effort as being of “such an enormous consequence” given the scope of the crisis, which adds up to the largest flow of refugees since World War II. He appeared to lament that security concerns “understandably” will keep him from visiting Ukraine.

Duda, who appeared with Biden on Friday, said the refugees are “guests.”

“We do not want to call them refugees. They are our guests, our brothers, our neighbors from Ukraine, who today are in a very difficult situation,” he said.

The U.S. has been sending money and supplies to aid the refugee effort. This week, Biden announced $1 billion in additional aid and said the U.S. would accept up to 100,000 refugees.

The U.S. and many of its allies have imposed multiple rounds of economic and other sanctions on Russian individuals, banks and other entities in hopes that the cumulative effect over time will force Putin to withdraw his troops.

Biden was scheduled to return to Washington after his speech in Warsaw on Saturday.