US President Joe Biden signed official ratification documents approving Sweden and Finland's accession to Nato on Tuesday after senators rebuked Russia by signing them off last week.
"In a moment when [Vladimir] Putin's Russia has shattered peace and security in Russia, when autocrats are changing the very foundation of a rule-based order, the strength of the Transatlantic Alliance, and America's commitment to Nato is more important than it has ever been," said Mr Biden, who was surrounded by the ambassadors of Sweden and Finland at the signing ceremony.
"Putin thought he could break us apart... he's getting exactly what he did not want. He wanted the Finland-isation of Nato, but instead he's getting the Nato-isation of Finland, along with Sweden."
In a rare display of unity, the upper chamber voted 95 to 1 in favour of supporting the two nations' bids to join the defensive military alliance. Sweden and Finland applied for Nato membership after Russia invaded Ukraine, defying warnings from Moscow not to seek membership.
Mr Biden celebrated the Senate's “historic vote” and lauded the prospective members' defensive capabilities.
Mr Biden encouraged the two Nordic nations to join the alliance when he hosted Finland's President Sauli Niinisto and Swedish Prime Minister Magdalena Andersson at the White House in May.
"Our alliance is closer than ever," he said. "It is more united than ever. And when Finland and Sweden bring the number of allies to 32, it will be stronger than ever."
Ms Andersson said at the time that Sweden would be best protected by Nato.
Sweden and Finland maintained close relationships with the alliance and regularly carried out joint exercises, shared information and participated in peacekeeping missions.
However, they are not protected by Article 5, the Nato defence clause that states an attack on one member is an attack on them all. Article 5 is considered to be the cornerstone of the alliance.
Sweden and Finland applied for Nato membership in May, ending a long-standing tradition of maintaining neutrality. Russian President Vladimir Putin has demanded that Russia halt its expansion efforts.
Accession must be ratified by the parliaments of all of Nato's 30 members. Ratification could take up to a year, but has already been approved by Germany, Canada and Italy.
Mr Biden delivered the protocols for Senate review last month.