Finland officially became the 31st member of the North Atlantic Treaty Organisation ( NATO ) on Tuesday, triggering a warning of counter-measures from Russia. 

The Finnish flag was raised at the Brussels headquarters of the world’s largest military alliance on Tuesday afternoon, almost a year after Helsinki officially applied to join. 

With the handing over of documents, the Nordic nation officially entered the world’s biggest security alliance, doubling its border with Russia. 

U.S. Secretary of State, Antony Blinken and NATO Secretary General, Jens Stoltenberg, were on hand as the Minister for Foreign Affairs, Pekka Haavisto, established Finland’s membership. 

“Finland has today become a member of the defence alliance NATO. The era of military non-alignment in our history has come to an end. A new era begins,” the Finnish presidency said in a statement. 

“Each country maximizes its own security. So does Finland. At the same time, NATO membership strengthens our international position and room for maneuver. As a partner, we have long actively participated in NATO activities. In the future, Finland will make a contribution to NATO’s collective deterrence and defence,” it added.

Mr Stoltenberg praised the raising of the Finish flag for the first time at the alliance headquarters in Belgium. 

“It will be a good day for Finland’s security, for Nordic security, and for NATO as a whole,” Mr Stoltenberg said. 

The move is a strategic political blow to Russian President Vladimir Putin who has always complained about NATO’s expansion toward Russia.

Russia warned that it would be forced to take “retaliatory measures” to address what it called security threats created by Finland’s membership. 

It has also warned it will bolster forces near Finland if NATO sends any additional troops or equipment to what will be its 31st member country. 

“We will strengthen our military capabilities in the west and northwest if NATO members deploy forces and equipment on Finnish territory,” Russia’s Deputy Foreign Minister, Alexander Grushko, told the Russian state news agency, RIA Novosti. 

Kremlin spokesperson, Dmitry Peskov, added that Finland’s accession will force Moscow to “take counter-measures to ensure our own security, both tactically and strategically.”