Finland, which shares a more than 800-mile long border with Russia, joined the NATO alliance on Tuesday, becoming the 31st member country.

The country’s membership was made official during a formal ceremony at NATO headquarters in Brussels on Tuesday.

Ahead of the official ceremony on Tuesday, NATO Secretary General Jens Stoltenberg said “it will be a good day for Finland’s security, for Nordic security, and for NATO as a whole.”

“By [Finland] become a full-fledged member, we are removing the room for miscalculation in Moscow about NATO’s readiness to protect Finland, and that makes Finland safer and stronger, and all of us safer,” Stoltenberg said.

Stoltenberg also told reporters “there will be no NATO troops in Finland without the consent of Finland.”

Finland applied for NATO membership in May, months after Russia invaded Ukraine. Sweden also applied, though the application process has been delayed because of objections from NATO members Turkey and Hungary.

Russian President Vladimir Putin has decried the expansion of NATO and threatened to take “retaliatory measures” in response to alleged security threats created by Finland’s membership in the military alliance. Russia has said it will strengthen its forces near Finland if the U.S.-led security alliance sends any additional troops or equipment to the country.

The Russian 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.” The foreign ministry claimed Finland’s membership marks “a fundamental change in the situation in Northern Europe, which had previously been one of the most stable regions in the world.”

Meanwhile, NoName057 (16), a pro-Russian hacker group, claimed responsibility for a denial-of-service attack against the website for Finland’s Parliament. The group said the attack was retaliation for Finland joining NATO, though its claim has not been independently verified.