Finland joins NATO, creating a new geopolitical nightmare for Putin, one that he sparked himself

• Finland has officially joined NATO, creating a nightmare scenario for Russia.
• NATO now has a powerful new member that has a long border with Russia.

Finland became the newest member of NATO on Tuesday, creating a nightmare scenario for Russia.

A ceremony and exchange of documents took place on Tuesday afternoon local time, completing the formal process.

Finland joining the Western military alliance is something that Russia has actively opposed, as it sees NATO as its greatest threat. But in many ways, Finland's membership is a direct result of Russia's own actions.

Russian President Vladimir Putin says the invasion of Ukraine, which started in February 2022, is necessary to prevent NATO from getting bigger and closer to Russia's borders.

But it was this that pushed Finland, and its neighbor Sweden, towards the alliance and to abandon their longstanding policies of neutrality.

Public support for membership soared in both countries, and they both sought membership in May 2022.

That was a historic shift, as the two countries had a decades-long neutrality policy, though they became NATO partner countries after the collapse of the Soviet Union.

Finland's former Prime Minister Alexander Stubb, who had long wanted his country to join NATO, told Insider last year that "Putin has only himself to thank, you know. I should probably be grateful to Putin."

Finland being a NATO member also brings the alliance physically closer to Russia.

Finland has more than 800 miles of land border with Russia. It joining NATO means that the length of the border between NATO countries and Russia more than doubles — taking it from around 750 miles to around 1,600.

Finland's membership was welcomed by its neighbors.

Estonia's president said that Finland's accession would "significantly strengthen the security of Baltic Sea region and all of NATO."

But Russia said in response that it will boost its military presence near their shared border.

Russia's Deputy Foreign Minister Alexander Grushko told Russian outlet RIA on Monday that they "will strengthen our military potential in the western and northwestern direction."

He added that in the event that resources of other NATO members are deployed in Finland, "we will take additional steps to reliably ensure Russia's military security," according to Reuters.

Russia threatened Finland and Sweden last year, saying that it would retaliate if they joined the military alliance.

Its statements became more measured over time, but Russia's ambassador to Sweden said last month that the two countries will become a "legitimate target" upon joining NATO.

Finland's membership had been delayed as Turkey, a NATO member country, held out on approving its membership. Turkey has accused Finland and Sweden of supporting groups that Turkey sees as terrorists.

But last month the country's president said that Finland had taken "authentic and concrete steps" to address his grievances.

Turkey's parliament approved a bill last Thursday that allowed Finland to join — though the country has not yet approved Sweden's bid.