Finland has formally joined NATO, its flag unfurling outside the military bloc’s Brussels headquarters, in a historic policy shift brought on by Russia’s invasion of Ukraine that drew a threat from Moscow of “countermeasures”.

Finland’s accession, ending seven decades of military non-alignment, roughly doubles the length of the border the North Atlantic Treaty Organisation shares with Russia and bolsters its eastern flank as the war in Ukraine grinds on with no resolution in sight.

Finland’s flag – a blue cross on a white background – was hoisted alongside those of the alliance’s 30 other members as a military band played in bright spring sunshine.

“For almost 75 years, this great alliance has shielded our nations and continues to do so today,” NATO Secretary-General Jens Stoltenberg declared at the ceremony on Tuesday.

“But war has returned to Europe and Finland has decided to join NATO and be part of the world’s most successful alliance.”

Stoltenberg earlier noted that Russian President Vladimir Putin had cited opposition to NATO’s eastward enlargement as one justification for invading Ukraine.

“He is getting exactly the opposite… Finland today, and soon also Sweden will become a full-fledged member of the alliance,” Stoltenberg said.

Finnish President Saul Niinisto said Finland’s most significant contribution to NATO’s common deterrence and defence would be to defend its own territory. There is still significant work to be done to co-ordinate this with NATO, he said.

“It is a great day for Finland and I want to say that it is an important day for NATO,” Niinisto said at a joint news conference with Stoltenberg.

The Kremlin said Russia would be forced to take “countermeasures.” Defence Minister Sergei Shoigu said Finland’s accession raised the risk of the Ukraine conflict escalating further.

In dropping non-alignment, Russia’s foreign ministry said, Finland was committing a dangerous historical mistake that would fray relations with Moscow and undo its status as a confidence-building presence in the Baltic Sea and Europe at large.

“This is now a thing of the past. Finland has become one of the small members of (NATO) that doesn’t decide anything, losing its special voice in international affairs. We are sure that history will judge this hasty step,” a ministry statement said.

Russia said on Monday it would strengthen its military capacity in its west and northwest in response to Finland joining NATO.

Ukraine hailed Finland’s step.

“I congratulate all the people of Finland,” President Volodymyr Zelenskiy said in his evening address. “Russian aggression clearly proves that only collective guarantees, only preventive guarantees, can be reliable.”

Finland’s era of strategic non-alignment began after the country repelled an attempted Soviet invasion during World War II and opted to maintain friendly relations with neighbouring Russia.

But the invasion of Ukraine in February 2022 prompted Finns to seek security under NATO’s collective defence pact, which states that an attack on one member is an attack on all.

Since the end of the Cold War three decades ago, Moscow has watched successive waves of NATO enlargement, and the issue was contentious even before the invasion of Ukraine.

NATO has repeatedly stressed that it is solely a defensive alliance and does not threaten Russia. Moscow says the funnelling of heavy weaponry to Ukraine by NATO countries since the war began proves the West is bent on destroying Russia.

Finland and its neighbour Sweden applied together last year to join NATO, but Sweden’s application has been held up by NATO members Turkey and Hungary.

Swedish Foreign Minister Tobias Billstroem told reporters it was Stockholm’s ambition to become a member at the NATO summit in Vilnius in July.

Turkey says Stockholm harbours members of what Ankara considers terrorist groups – which Sweden denies – and has demanded their extradition as a step toward ratification.

Hungary cites grievances over Swedish criticism of Prime Minister Viktor Orban’s record on democracy and rule of law.