In a speech delivered on the eve of an important meeting of the 28 NATO nations in Wales, President Obama vowed to defend the sovereignty of all members of the alliance.
Obama spoke in Estonia, one of the Baltic countries that has seen Russia's military involvement in Ukraine as a threat to its own sovereignty.
"You lost your independence once," Obama said. "But with NATO you will never lose it again."
The speech sent a clear warning to Russia that the United States will back any NATO member who sees its territorial integrity challenged with the full force of the U.S. military.
But President Obama also delivered a staunch defense of democracy. He said the current crisis in Ukraine represents a "moment of testing" for the alliance, challenges its basic beliefs and "undermines an international order."
However, Obama said, "more than any other form of government ... only democracy rooted in the sanctity of the individual can deliver real progress."
He added that democracy and freedom will always win in history because "basic human yearnings," which are at the heart of NATO democracies, "do not go away."
"They burn in every human heart," Obama said.
He added: "The currents of history ebb and flow, but over time they flow toward freedom."