"We're living in a time when democracy is more at risk across the world than at any point since the end of World War II," he said in the French town of Colleville-sur-Mer.
"Isolationism was not the answer 80 years ago and is not the answer today," he added.
Paying homage to the US and other Allied soldiers who fought on the Normandy beaches in June 1944, Biden stressed that "strong alliances" were key.
"What the Allies did together 80 years ago far surpassed anything we could have done on our own," he said.
"It was a powerful illustration of how alliances -- real alliances -- make us stronger –- a lesson that I pray we Americans never forget."
He said the United States would not abandon Ukraine, which has been battling invading Russian forces for over two years.
"We will not walk away because if we do Ukraine will be subjugated and it will not end there," Biden said.
"Ukraine's neighbours will be threatened, all of Europe will be threatened," he added, describing President Vladimir Putin as a "tyrant bent on domination".
"There are things that are worth fighting and dying for. Freedom is worth it. Democracy is worth it," he said.
He praised NATO as "the greatest military alliance in the history of the world."
"NATO is more united than ever and even more prepared to keep the peace," he said in a thinly veiled message to Putin.
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