President Joe Biden says the US commitment to the NATO alliance is "unshakeable" and promises to observe the principle that an attack on one member is an attack on all.
His statement on Friday is at odds with his predecessor, Donald Trump, who called the 30-member alliance outdated and at one point suggested Washington could withdraw.
"The United States is fully committed to our NATO alliance, and I welcome your growing investment in the military capabilities that enable our shared defenses," Biden told an online session of the Munich Security Conference on Friday.
"An attack on one is an attack on all. That is our unshakeable vow."
Trump administration officials had publicly hammered, and sought to shame, Germany and other NATO members for not meeting a target of spending two per cent of their gross domestic output on defense.
Biden's comments signalled a different approach - one welcomed by European leaders.
"America is back, the trans-Atlantic alliance is back," Biden told the security conference after his first virtual meeting with Group of Seven world leaders.
"I know the past few years have strained and tested our trans-Atlantic relationship but the United States is determined - determined - to re-engage with Europe, to consult with you, to earn back our position of trusted leadership," he said.
"Welcome back, America," said European Council President Charles Michel, effectively summing up the mood of the Munich conference.
Biden said the US military was conducting a comprehensive review of its military posture around the world, but he had lifted orders to withdraw US troops from Germany - another decision by the Trump administration that had shocked allies.
In addition, Biden said he had lifted a cap imposed by the previous administration on the number of US forces that could be based in Germany.
German Defence Minister Annegret Kramp-Karrenbauer welcomed Biden's announcement.
"We clearly understand this confirmation of America's commitment to European security, a strong NATO, and a strategically unified West," she told Reuters.
"This signal will be noticed and well understood," Kramp-Karrenbauer said. "It is now up to us to take the hand that Washington has reached out with."