The Nordic country’s membership doubles Russia’s border with the world’s biggest security alliance and represents a major change in Europe’s security landscape: The nation adopted neutrality after its defeat by the Soviets in World War II. But its leaders signaled they wanted to join the alliance just months after Russian President Vladimir Putin’s invasion of Ukraine sent a shiver of fear through Moscow’s neighbors.

The move is a strategic and political blow to Putin, who has long complained about NATO’s expansion toward Russia and partly used that as a justification for the 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 to what is its 31st member country.

The alliance says it poses no threat to Moscow.

Neighboring Sweden, which has avoided military alliances for more than 200 years, has also applied. But objections from NATO members Turkiye and Hungary have delayed the process.

Alarmed by Moscow’s invasion of Ukraine last year, Finland, which shares a 1,340 kilometer (832 mile) border with Russia, applied to join in May, setting aside years of military non-alignment to seek protection under the organization’s security umbrella.

“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,” US Secretary of State Antony Blinken said just before accepting the documents that made Finland’s membership official.

The US State Department is the repository of NATO texts concerning membership.

Earlier, Russia’s Foreign Ministry said the country “will be forced to take military-technical and other retaliatory measures to counter the threats to our national security arising from Finland’s accession to NATO.”

It said Finland’s move marks “a fundamental change in the situation in Northern Europe, which had previously been one of the most stable regions in the world.”

Kremlin spokesman Dmitry Peskov, meanwhile, 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 has no territorial disputes with Finland.

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

NATO Secretary-General Jens Stoltenberg earlier said that no more troops would be sent to Finland unless it asked for help.

“There will be no NATO troops in Finland without the consent of Finland,” NATO Secretary-General Jens Stoltenberg told reporters at the alliance’s headquarters in Brussels a few hours before the country joins.

The country 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.

But Stoltenberg refused to rule out the possibility of holding more military exercises there and said that NATO would not allow Russia’s demands to dictate the organization’s decisions.

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

Meanwhile, Finland’s Parliament said that 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 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 party 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.

Finland’s entry, to be marked with a flag-raising ceremony at NATO headquarters, falls on the organization’s very own birthday, the 74th anniversary of the signing of its founding Washington Treaty on April 4, 1949. It also coincides with a meeting of the alliance’s foreign ministers.

Finland’s president, foreign and defense ministers will take part in the ceremony.

Turkiye became the last NATO member country to ratify Finland’s membership protocol on Thursday. It will hand over the document officially enshrining that decision to US Secretary of State Antony Blinken before the ceremony.

Finland’s membership becomes official when its own foreign minister hands over documents completing its accession process to Blinken. The US State Department is the repository of NATO texts concerning membership.