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

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.

Finland’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.”