Banners displaying the NATO logo are placed at the entrance of its headquarters in Brussels, Belgium. Reuters

Last year, the Kremlin's all-out invasion of Ukraine upended Europe's security landscape and prompted Finland — and its neighbour Sweden — to drop decades of non-alignment.

Awkward allies Turkey and Hungary, for different reasons of their own, delayed Finland's bid to come under the NATO umbrella — and Stockholm's progress remains blocked.

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But last week, the Turkish parliament voted to clear Finland's last hurdle.

Completing the ratification in well under a year still makes this the fastest membership process in the alliance's recent history.

Finnish reservists of the Guard Jaeger Regiment fill their assault rifle magazines in Helsinki, Finland. AFP

All that remained were Tuesday's highly choreographed formalities at NATO headquarters.

Finland's foreign minister will hand over the formal accession papers to US Secretary of State Antony Blinken, the keeper of NATO's founding treaty.

Then the country's blue-and-white flag will be raised next to those of its new allies, between those of Estonia and France, in front of the gleaming headquarters in Brussels.

"This is really an historic day. It's a great day for the alliance," NATO chief Jens Stoltenberg said.

Joining NATO places Finland under the alliance's Article Five, the collective defence pledge that an attack on one member "shall be considered an attack against them all".

Finland's President Sauli Niinisto (right) meets with NATO chief Jens Stoltenberg in Naantali, Finland. File/AP

This was the guarantee Finnish leaders decided they needed as they watched Russian President Vladimir Putin's devastating assault lay waste to swathes of Ukraine.

Putin gets more NATO

"President Putin went to war against Ukraine with a clear aim to get less NATO," Stoltenberg said. "He's getting the exact opposite."

Invaded by its giant neighbour the Soviet Union in 1939, Finland — which has a 1,300-kilometre (800 mile) border with Russia — stayed out of NATO throughout the Cold War.

Now its membership brings a potent military into the alliance with a wartime strength of 280,000 and one of Europe's largest artillery arsenals.

And its strategic location bolsters NATO's defences on a border running from the vulnerable Baltic states to the increasingly competitive Arctic.

NATO was created as a counterweight to the Soviet Union at the onset of the Cold War era that began immediately after the Allies defeated Nazi Germany.

Finnish reservists of the Guard Jaeger Regiment stand at a shooting range in Helsinki, Finland. File/AFP

The bloc has gone through waves of expansion that brought it ever closer to Russia's borders.

NATO's reach into eastern and southern European countries that were once under Moscow's effective control infuriated the Kremlin and strained its relations with Washington.

Putin cited the threat of NATO expanding into Ukraine as one of his main reasons for launching the war 13 months ago.

At first, the Kremlin appeared to play down the significance of the alliance's border advancing to touch a new stretch of Russia's northwestern frontier.

But it has pledged to bolster its forces and stepped up diplomatic rhetoric in recent weeks, describing Finland and Sweden as a "legitimate target" if they join NATO.

Putin has also announced plans to deploy tactical nuclear weapons in Russia's neighbour Belarus.

Agence France-Presse