Finland said on Thursday it would apply to join NATO "without delay", with Sweden expected to follow, as Russia's invasion of Ukraine looked set to bring about the very expansion of the Western military alliance that Vladimir Putin aimed to prevent.
The decision by the two Nordic countries to abandon the neutrality they maintained throughout the Cold War would be one of the biggest shifts in European security in decades. Moscow called Finland's announcement a direct threat to Russia, and threatened retaliation, including unspecified "military-technical" measures.
It came even as Russia's war in Ukraine was suffering another big setback, with Ukrainian forces driving Russian troops out of the region around the second largest city Kharkiv, the fastest Ukrainian advance since forcing Russia to withdraw from the capital and northeast more than a month ago.
NATO Secretary-General Jens Stoltenberg said the Finns would be "warmly welcomed" and promised a "smooth and swift" accession process.
Russia's foreign ministry said Moscow would be forced to take 'retaliatory steps, both of a military-technical and other nature' giving no further details
Finland and Sweden are the two biggest EU countries yet to join NATO. Finland's 1,300-km (800-mile) border will more than double the length of the frontier between the U.S.-led alliance and Russia, putting NATO guards a few hours' drive from the northern outskirts of St Petersburg.
"Finland must apply for NATO membership without delay," President Sauli Niinisto and Prime Minister Sanna Marin said in a joint statement, hoping steps to take the decision would "be taken rapidly within the next few days".
Asked whether Finland's accession posed a direct threat to Russia, Kremlin spokesman Dmitry Peskov said: "Definitely. NATO expansion does not make our continent more stable and secure.
"This cannot fail to arouse our regret, and is a reason for corresponding symmetrical responses on our side," Peskov added.
Russia's foreign ministry said Moscow would be forced to take "retaliatory steps, both of a military-technical and other nature", giving no further details. Russian officials have spoken in the past about potential measures including stationing nuclear-armed missiles on the Baltic Sea.
Asked on Wednesday if Finland would provoke Russia by joining NATO, Niinisto said: "My response would be that you caused this. Look at the mirror."
Five diplomats and officials told Reuters that NATO allies expect both countries to be granted membership quickly, paving the way for an increased troop presence in the Nordic region to defend them during a one-year ratification period.
Putin, Russia's president, cited NATO's potential expansion as one of the main reasons he launched a "special military operation" in Ukraine in February.
NATO describes itself as a defensive alliance, built around a treaty declaring that an attack on one member is an attack on all, granting U.S. allies the protection of Washington's superpower might including its nuclear arsenal.
Moscow regards that as a threat to its security. But Putin's decision to invade Ukraine has changed Nordic public opinion, with political parties that had long backed neutrality now embracing the view that Russia is a menace.
Finland in particular has centuries of uneasy history in Russia's shadow. Ruled by the Russian empire from 1809-1917, it fought off Soviet invasions on the eve of World War Two, and accepted some Soviet influence as the price of avoiding taking sides in the Cold War. Since it and Sweden joined the EU in 1995, they have aligned more firmly with the West.