WASHINGTON — President Biden signed a measure on Tuesday that would expand NATO to include Sweden and Finland in an effort to bolster the Western alliance nearly six months after President Vladimir V. Putin of Russia launched his invasion of Ukraine.
“It’s critical now to deter threats before they harm our people, our allies and our interests,” Mr. Biden said from the White House. “It’s how we address instability and aggression, with allies at our side amplifying the capacity to respond effectively.”
“Sweden and Finland have strong Democratic institutions, strong militaries and strong and transparent economies,” the president added. “They will meet every NATO requirement — we’re confident of that — and it will make our alliance stronger.”
All 30 current members of NATO must approve of the addition of the two countries, and more than 20 have already done so. The NATO expansion gained significant momentum after Turkey lifted a veto on adding Sweden and Finland following a set of commitments by the two countries that they would act against terrorism.
The turnaround by President Recep Tayyip Erdogan of Turkey was a significant accomplishment in the Biden administration’s quiet diplomatic push to unify the West around countering Russia’s assault on Ukraine.
The expansion of NATO has received overwhelming support among Democrats and Republicans in a deeply divided Washington. Last week, the Senate voted 95 to 1 to give its approval, with only Senator Josh Hawley, Republican of Missouri, opposing the move.
Only the Senate has the power to approve treaties, but last month, the House passed a nonbinding resolution in support of Finland and Sweden’s accession to NATO in a lopsided vote of 394 to 18.
Prime Minister Magdalena Andersson of Sweden and President Sauli Niinisto of Finland visited the White House in May, and Mr. Biden said on Tuesday that he spoke to them on the phone before the signing ceremony.
“Today we see all too clearly how NATO remains an indispensable alliance,” Mr. Biden said, adding, “When Finland and Sweden bring the number of allies to 32, we’ll be stronger than ever.”
Democrats have argued that adding Finland and Sweden to NATO would reduce the burden on the United States and other allies that are assisting Ukraine. The approval in Washington was another pivot away from the foreign policy of President Donald J. Trump, who openly criticized the alliance.