As Russia’s invasion of Ukraine threatened to bring about the very enlargement of the Western military alliance that Vladimir Putin sought to prevent, Finland announced on Thursday that it would apply to join NATO “without delay,” with Sweden anticipated to follow suit.

The decision by the two Nordic countries to renounce their Cold War-era neutrality would be one of the most significant developments in European security in decades. The Kremlin was enraged by Finland’s announcement, which it saw as a direct threat to Russia and warned an unspecified response.

It happened as Russia’s campaign in Ukraine suffered yet another blow, with Ukrainian forces driving Russian troops out of the territory surrounding Kharkiv, the fastest Ukrainian advance since forcing Russia to retreat from the capital and northeast over a month ago.

Jens Stoltenberg, NATO Secretary-General, said the Finns will be “warmly welcomed” and pledged a “smooth and speedy” admission process.

The two largest EU countries that have yet to join NATO are Finland and Sweden. Finland’s 1,300-kilometer (800-mile) border with Russia will more than treble the length of the NATO-Russian border, putting NATO troops just a few hours’ drive from St Petersburg’s northern fringes.

In a joint statement, President Sauli Niinisto and Prime Minister Sanna Marin said, “Finland must apply for NATO membership without delay.” “We hope that the remaining national procedures required to make this decision will be taken quickly in the coming days.”

“Definitely,” said Kremlin spokesman Dmitry Peskov when asked if Finland’s entry constituted a direct danger to Russia. NATO expansion does not improve the stability and security of our continent.

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