Ukrainian President Volodymyr Zelensky told the World Economic Forum on Tuesday that it is time for Europe to remain strong and united as the United States installs a new president who is skeptical of global alliances and continued aid to Ukraine.

“Europe must shape history for itself and its allies,” he told dozens of government leaders at the annual Alpine summit in Davos, Switzerland.

Mr Zelensky spoke less than 24 hours after President Trump was sworn in for the second time. But already, he said, “most of the world is thinking about what will happen to their relationship with America.”

Although America remains an “indispensable ally” that can help end Russia’s war of aggression, Mr. Zelensky warned European leaders that they do not have the luxury of being separated by an ocean from the revanchist regime in the Kremlin.

Therefore, he said, Europe must ensure that its voice is heard in the talks to ensure not only a quick end to the fighting in Ukraine, but also a just and lasting peace.

“Will President Trump listen to Europe or will he negotiate with Russia and China without Europe?” Mr. Zelensky asked. “Europe must learn how to take care of itself.”

He said he had a good relationship with Mr Trump and recalled telling the US president that Ukraine wanted to be his partner in ending the bloodshed. No one, he said, wants the war to end more desperately than the Ukrainians.

Mr. Trump has offered little insight into how he intends to fulfill his campaign pledge to end the fighting, and there is little indication that Moscow is willing to budge from its maximalist demands that would leave Ukraine split in two and militarily neutered.

Mr. Zelensky offered sobering statistics about how vulnerable Europe would be without American help and described the grim scenario he said nations across the continent could face if Russia achieves its stated war goal of castrating Ukraine.

For example, despite a far smaller economy than the European Union, Moscow can produce significantly more weapons and ammunition, he said. Russia can also send 1.5 million troops, he said, while Ukraine currently has more than 800,000 servicemen and women. The biggest power in Western Europe is France with about 200,000 soldiers.

“If it takes 5 percent of GDP to cover defense, then so be it, 5 percent is,” he said, referring to the amount Mr. Trump called on all NATO allies to spend.

Earlier this month, the conservative-leaning American Enterprise Institute estimated that if Russia prevails in its bid to subdue Ukraine, the United States will need to bolster its own military by spending an additional $808 billion over five years to protect Europe.

“These resources would be required on top of the currently planned defense budget during that same period and would be used to build defense capacity, capabilities, and posture to ensure U.S. security and meet current commitments to NATO in a new, more dangerous strategic environment,” the the institute states in the report.

The cost to Europe could be even higher, Mr. Zelensky said.

Without strong security guarantees for Ukraine — guarantees he said must include some combination of NATO membership, long-range weapons to deter aggression and allied troops deployed in Ukraine to deter aggression — he said Russia would continue to threaten all countries President Vladimir V. Putin considers it to be within Russia’s self-determined sphere of influence.

“He will bring us into the world that lives in his head,” Mr. Zelensky said.

North Korean troops, he said, are currently engaged in fighting in the Kursk region of Russia, which is closer to Davos than to Pyongyang. Moscow and Tehran have just signed a security agreement cementing an alliance driven by a shared desire to challenge the West. China continues to help prop up the Russian economy.

“Who do they make such deals against?” said Mr. Zelenski. “Against you, against us. Against Europe, against America.”

“Russia is turning into a version of North Korea — a country where human life means nothing, but they have nuclear weapons and a burning desire to make life miserable for their neighbors,” he said.

He said he thought Mr. Trump finally understands why it is in America’s interest to fight nations that are clearly committed to undermining US power and influence.

“There are some voices around Trump that are delivering disinformation or pro-Russian messages,” he said.

“It is very important that Europe is vocal,” he said, “so that it clearly understands the details and the risks.”