Under an agreement with the Soviet Union, Finland stayed outside the alliance, which was created to contain Russia after World War II. It remained independent in the post-Soviet era even after joining the European Union and growing ever closer with the West. Until now, Sweden had kept to more than 200 years of neutrality.
But that posture has been quickly abandoned after Mr. Putin’s decision in February to invade Ukraine, which is not a NATO member. Both Finland and Sweden suddenly realized that the threat from Russia had changed and that their status as a bystander to great-power conflict was now a huge risk.
The speed of the reversal has been so great that there has been virtually none of the debate that took place after the fall of the Berlin Wall in 1989, when even some of Washington’s most experienced Cold War diplomats warned that the more Russia felt encircled, the higher the chances that it might eventually lash out, especially if the effort to integrate the country with the West failed.
On Wednesday, Mr. Sullivan said that Mr. Biden had asked his national security officials whether they backed the addition of Finland and Sweden to the alliance and that they had “emphatically supported” the move in a unanimous fashion.
The Rose Garden ceremony deliberately contained echoes of a state visit, complete with a military band. Mr. Biden characterized the move to usher Finland and Sweden into the alliance as almost a formality, noting that both countries had contributed forces to conflicts in Kosovo, Afghanistan and Iraq — the major NATO commitments of the past 20 years — and that they were strong democracies that “meet every NATO requirement and then some.”
Mr. Biden argued that the two countries would add to the alliance’s firepower.
Finland has a sophisticated military that runs complex operations to track Russian activity in the seas of Northern Europe and spends heavily on modern equipment. Sweden is a more difficult case: It dismantled some of its military power and, as Ms. Andersson conceded, would have to reorient its budget to spend 2 percent of gross domestic product on defense, the target for NATO members.
But for the United States, the primary utility of having the Nordic countries join the alliance is the message sent to Mr. Putin. In December, the Russian president demanded that the United States and NATO sign a treaty that would withdraw forces from former Soviet states and that they restrict training activities and the placement of arms.