The North Atlantic Treaty Organization has a new Nordic member.

Finland formally joined the military alliance Tuesday, expanding the bloc’s border with Russia by more than 800 miles and becoming the 31st member since its founding 74 years ago.

“On this very day, in 1949, the Washington Treaty, NATO’s founding treaty, was signed in Washington and it is hard to imagine a better way of celebrating our anniversary than to have Finland becoming a full member of the Alliance,” a jubilant NATO Secretary General Jens Stoltenberg told reporters in Brussels.

Finland had adopted neutrality since its defeat by the Soviet Union during World War II, but it moved to join the alliance after Russia invaded Ukraine last year.

The move is a strategic and political blow to Russian President Vladimir Putin, who has long complained about NATO’s expansion toward his country and partly used that as a justification for the Ukraine invasion.

Russia warned that it would be forced to take “retaliatory measures” to address what it called security threats created by Finland’s membership. It had also warned it would bolster forces near Finland if NATO sends any additional troops or equipment.

Kremlin spokesman Dmitry Peskov said Tuesday that Finland’s membership reflects the alliance’s anti-Russian course and warned that Moscow will respond depending on what weapons NATO allies place there.

But Peskov also sought to play down the impact, noting that Russia currently has no territorial disputes with Finland.

It’s not clear what additional military resources Russia could send to the Finnish frontier. Moscow has deployed the bulk of its most capable military units to Ukraine.

The alliance has insisted it poses no threat to Russia.

“There will be no NATO troops in Finland without the consent of Finland,” said Stoltenberg, while refusing to rule out the possibility of holding more military exercises there.

“We are constantly assessing our posture, our presence. We have more exercises, we have more presence, also in the Nordic area,” Stoltenberg added.

With its accession to membership, Finland is now protected by what Stoltenberg called NATO’s “iron-clad security guarantee,” under which all member countries vow to come to the defense of any ally that comes under attack.

President Biden issued a statement hailing the expansion of NATO, saying: “When Putin launched his brutal war of aggression against the people of Ukraine, he thought he could divide Europe and NATO. He was wrong. Today, we are more united than ever. And together—strengthened by our newest Ally Finland—we will continue to preserve transatlantic security, defend every inch of NATO territory, and meet any and all challenges we face.”

“I’m tempted to say this is maybe the one thing that we can thank Mr. Putin for because he once again here precipitated something he claims to want to prevent by Russia’s aggression, causing many countries to believe that they have to do more to look out for their own defense and to make sure that they can deter possible Russian aggression going forward,” Secretary of State Antony Blinken said just before accepting the documents that made Finland’s membership official.

Neighboring Sweden, which has avoided military alliances for more than 200 years, has also applied for membership, though objections from Turkey and Hungary have delayed the process.

In his statement, Biden called on the governments in Ankara and Budapest to ratify Stockholm as the 32nd NATO member “without delay.”

Meanwhile, Finland’s Parliament said its website was hit with a so-called denial-of-service attack, which made the site hard to use, with many pages not loading and some functions not available.

A pro-Russian hacker group known as NoName057 (16) claimed responsibility, saying the attack was in retaliation for Finland joining NATO.

The claim could not be immediately verified.

The hacker group, which has reportedly acted on Moscow’s orders, has taken part in a slew of cyberattacks on the US and its allies in the past.

Finnish public broadcaster YLE said the same group hit the Parliament’s site last year.

with Post wires