U.S. President Joe Biden offered an unequivocal commitment to defend fellow NATO countries on Monday.
"Article 5 we take as a sacred obligation."
Article five is the mutual-defense clause of the military alliance.
"I just want all of Europe to know that the United States is there."
The members are in Brussels for an annual summit.
Biden sought to rally Western allies to support a U.S. strategy to contain China's military rise, and present a united front in the face of Russian aggression.
Allied leaders are concerned about Russia's recent military build-up near Ukraine, as well as its covert and cyber attacks to undermine Western states.
Moscow denies any wrongdoing.
Beijing, while not seen as an outright adversary, is no longer seen as a benign trading partner.
Here's how NATO Secretary-General Jens Stoltenberg put it:
"China does not share our values. We see how they crack down on democratic protests in Hong Kong. [FLASH] All of this matters for our security."
Another topic at the NATO summit is the handover of power in Washington.
Biden's predecessor disparaged the alliance as obsolete, harangued members to pay more for mutual defense, and sometimes appeared to cozy up to their number one adversary, Moscow.
Dutch Prime Minister Mark Rutte seemed relieved to see the U.S. administration once again committed to the principles of the defense pact.
"I also was able to work with Trump, of course it was a bit more awkward, sometimes. [FLASH] But I think with (U.S. President) Joe Biden, it's more natural again. Thank you."
Biden will meet Russian President Vladimir Putin in Geneva on Wednesday.