NORMANDY — President Biden observed the 80th anniversary of D-Day on the beaches of Normandy on Thursday by asserting that the allied effort to stand up to Russia’s invasion of Ukraine is a direct extension of the battle for freedom that raged across Europe during World War II.

Addressing 180 surviving veterans of the Normandy operation and thousands of other guests, Mr. Biden said that the world must defeat another “tyrant bent on domination” and meet “the test of ages” to defend Ukraine — just as the heroes who stormed the beaches and dropped behind enemy lines did eight decades ago.

“Isolation was not the answer 80 years ago and is not the answer today,” Mr. Biden said, with World War II veterans seated in wheelchairs behind him. “We know the dark forces that these heroes fought against 80 years ago. They never fade. Aggression and greed, the desire to dominate and control, to change borders by force — these are perennial. The struggle between dictatorship and freedom is unending.”

In an energetic address, Mr. Biden declared that “NATO is more united than ever” and insisted that the alliance would stand by Ukraine in its own hour of need just as the United States had stood by Europe against the Nazis.

“We will not walk away,” Mr. Biden said. “Because if we do, Ukraine will be subjugated and it will not end there.”