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Sweden announced on Monday that it will apply for membership of Nato, joining its neighbour Finland in tearing up decades of military neutrality in a historic shift prompted by Russia's invasion of Ukraine.
The decision announced by Prime Minister Magdalena Andersson comes despite threats of retaliation from the Kremlin against the two countries poised to join the alliance.
Russian President Vladimir Putin said on Monday that the expansion of Nato, which Moscow has long sought to prevent, will "certainly provoke our response".
But an undeterred Sweden and Finland plan to formally submit their applications together in the coming days, Ms Andersson said.
A brief official statement said Nato membership was the best way to protect Sweden's security "in light of the fundamentally changed security environment following Russia’s invasion of Ukraine".
"Sweden needs formal security guarantees that come with membership in Nato," said Ms Andersson, who as recently as her inaugural address last November had opposed joining the alliance. "We are leaving one era and beginning another."
However, she said Sweden did not want permanent Nato military bases or nuclear weapons on its territory if its membership was approved.
The announcement came a day after Ms Andersson's ruling Social Democrats dropped their longstanding opposition to joining Nato, paving the way for a historic membership bid.
A debate among the eight parties in Sweden's parliament on Monday showed broad support for moving under Nato's umbrella, with only two small leftist parties expressing opposition. Finland's leaders backed Nato membership on Friday.
Nordic neighbours Iceland, Denmark and Norway said they would seek a swift accession process and come to Sweden's aid if foreign powers try to meddle before it has formally joined.
Nato's Secretary General Jens Stoltenberg, and leading powers including the US, Britain and Germany, have said they will welcome the new applicants. However, Nato member Turkey presented a potential obstacle by saying it had reservations.
Ms Andersson said Sweden could face "attempts to scare and divide us" before it becomes a full member, but said the process should not take more than a year.
Sweden and Finland already co-operate militarily and diplomatically with Nato and took part in recent drills in Europe's high north. Germany described them as "members already, just without membership cards".
But full membership means Sweden and Finland would be covered by Nato's Article 5 security guarantee, meaning western powers would promise to use their military might to protect them from invasion.
The two nations had previously sought to stay out of any such confrontation by remaining militarily neutral, but the onslaught on Ukraine prompted a change of heart in Stockholm and Helsinki.
Mr Putin views Nato expansion as a prime security grievance and had sought to curb any further enlargement in the stand-off over Ukraine, which culminated in the invasion.
"Russia's decision to start this war against Ukraine will not only go down in history for the massive suffering, but will also be remembered as a spectacular miscalculation," said Swedish opposition leader Ulf Kristersson.
Austrian Foreign Minister Alexander Schallenberg, whose country is not part of Nato, said at a meeting of European diplomats on Monday that Mr Putin's strategehas blown back into his face," said
Finland's accession would double the length of Nato's frontier with Russia. Sweden has no land border with Russia, but officials say the country's security environment has deteriorated since the invasion of Ukraine.
A report published by Sweden's government this month said the war in Ukraine showed that friendly countries outside Nato could not expect military intervention on their behalf. It added that it was "not politically, financially or militarily realistic" to secure alternative guarantees.
Sweden last fought a war in 1814 and pursued a policy of neutrality during the two world wars. It joined the EU in 1995, along with fellow neutral states Finland and Austria.