Sweden on Monday officially announced it will apply for NATO membership as a deterrent against Russian aggression, entering a "new era" and reversing two centuries of military non-alignment.
"The government has decided to inform NATO that Sweden wants to become a member of the alliance," Prime Minister Magdalena Andersson told reporters a day after neighbouring Finland made a similar announcement.
"We are leaving one era and beginning another," Andersson said of the dramatic turnaround of her country's position less than three months after Russia's invasion of Ukraine.
Sweden's NATO ambassador would "shortly" inform NATO, she said.
Sweden and Finland have both expressed a desire to act in lockstep on NATO membership. They are expected to submit their applications jointly this week.
Russian President Vladimir Putin on Monday warned that NATO's expansion may trigger a response from Moscow.
It poses "no direct threat for us... but the expansion of military infrastructure to these territories will certainly provoke our response," Putin said during a televised summit meeting of the Collective Security Treaty Organisation, a Moscow-led military alliance.
Andersson acknowledged Sweden would be "vulnerable" in the interim period before its application is ratified.
Stockholm has received security assurances from several key partners, including the United States, Britain, Germany, France and the Nordic countries, she added.
Sweden's Supreme Commander of the Armed Forces, Micael Byden, said the risk of a Russian military attack was "pretty low", but the country could expect to see "disinformation campaigns, cyberattacks, sabotage, (and) subversion".
Sweden's announcement was expected after Andersson's Social Democratic party on Sunday backed membership, in a dramatic U-turn. Sweden had opposed the idea since the birth of the Western military alliance.
The premier had earlier in the day consulted parliament by convening a debate, though lawmakers did not vote on the issue.
Six of eight parties in parliament, constituting a very broad majority, are in favour of joining. Swedish public support has also risen dramatically to around 50 percent -- with about 20 percent against.
In Helsinki, support has surged even more dramatically, with more than three-quarters of Finns in favour of membership, almost triple the level seen before the war in Ukraine began on February 24.
Finnish lawmakers on Monday launched a marathon debate with over 150 of 200 MPs asking to speak, following a NATO membership proposal presented on Sunday by President Sauli Niinisto and Prime Minister Sanna Marin.
"Our security environment has fundamentally changed," Marin told parliament.
"The only country that threatens European security, and is now openly waging a war of aggression, is Russia", she said.
Finland, which shares a 1,300-kilometre (800-mile) border with Russia, has a long shared history with Russia.
It spent more than a century as part of the Russian empire until it gained independence in 1917. Finland was then invaded by the Soviet Union in 1939.
Finns put up a fierce fight during the bloody Winter War, but were ultimately forced to cede a huge stretch of their eastern Karelia province in a peace treaty with Moscow.
An overwhelming majority of Finland's 200 MPs -- at least 85 percent -- back the decision to join NATO.
During the debate in Sweden's parliament, Andersson acknowledged that Sweden's decision to join NATO was closely tied to Finland's.
As the only country in the Baltic Sea region outside of NATO, Sweden would find itself "in a very vulnerable position", she told parliament.
She also stressed Sweden's "extensive military cooperation" with Finland.
If Sweden doesn't join, and "Finland as a NATO member focuses more on its cooperation with NATO countries, Sweden's defence capability decreases at a time when it instead needs to be strengthened."
"The best thing for our country's security is therefore for Sweden to apply for membership in NATO and to do it together with Finland," she said.
Andersson later said "it shouldn't take more than a year" for the alliance's 30 members to unanimously ratify Sweden's membership application.
NATO has said the two countries would be welcomed "with open arms", but Turkish President Recep Tayyip Erdogan has expressed last-minute objections.
Ankara is angered by what it considers to be the two countries' leniency, in particular Stockholm's, towards the Kurdistan Workers' Party (PKK), which is on the EU's list of terrorist organisations.
Swedish Defence Minister Peter Hultqvist said Monday Sweden was sending a delegation to Turkey for talks with officials.