Finland joins NATO, doubling alliance's border with Russia

Finland joined the North Atlantic Treaty Organisation (NATO) security alliance as the 31st member on Tuesday, and its flag will soon be raised at the alliance's headquarters.

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

Finland's membership represents a major change in Europe's security landscape: The country adopted neutrality after its defeat by the Soviets in World War II. But its leaders signalled 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 neighbours.

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. The alliance says it poses no threat to Moscow.

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.

Neighbouring 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.

Finland's membership became official when its foreign minister handed over documents completing its accession process to US Secretary of State Antony Blinken.

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