Finland became NATO’s 31st member on Tuesday in a historic expansion that will double the size of its military alliance with Russia.

The country’s accession will be completed during a formal ceremony at NATO headquarters in Brussels late Tuesday, marking a major shift in the security landscape in northeastern Europe, adding 1,300 kilometers (830 miles) to the alliance’s border with Russia.

Before this Tuesday, Russia shared about 1,215 kilometers of land border with five NATO members.

The merger ends months of diplomatic impasse and marks a turning point for the Nordic nation, which abandoned decades of neutrality when it announced its intention to join the alliance a year ago.

Both Finland and Sweden re-evaluated their positions after Russian President Vladimir Putin sent troops to Ukraine in February 2022.

However, Türkiye continues to block Sweden’s request. NATO has an open-door policy, meaning any country can be invited to join the alliance. But under the accession rules, any member state can veto the entry of a new country.

Following a unanimous vote in Turkey’s parliament to approve Finland’s NATO membership last week, Finland’s President Sauli Niinisto declared his country “ready to join NATO.”

“All 30 NATO members have now approved Finland’s accession. I would like to thank each and every one of them for their trust and support,” he added. “Finland will be a strong and capable ally, committed to the security of the alliance.”

“We look forward to welcoming Sweden to join us as soon as possible,” the Finnish president added.

CNN’s Ivana Kotasova and Tara John contributed reporting.