Sweden and Finland formally apply to join NATO after Russia invaded Ukraine

• Sweden and Finland ended decades of military nonalignment in Europe and applied to join NATO.
• Their decision Wednesday came after Russia invaded Ukraine, which isn't a NATO member, in February.

Sweden and Finland have formally applied to join the North Atlantic Treaty Organization, ending decades of neutrality and signaling a massive shift in Europe's security landscape.

The countries' ambassadors to NATO — Klaus Korhonen and Axel Wernhoff — handed in their applications to NATO Secretary-General Jens Stoltenberg at the alliance's headquarters in Brussels on Wednesday morning.

NATO welcomed the applications.

"This is a historic moment, which we must seize," Stoltenberg said Wednesday, Reuters reported.

"I warmly welcome requests by Finland and Sweden to join NATO. You are our closest partners, and your membership in NATO will increase our shared security."

NATO is a military alliance among the US, Canada, and more than two dozen European countries that have agreed to defend any alliance member that is attacked.

The move by Sweden and Finland, which stayed militarily nonaligned throughout the Cold War and beyond, to join the bloc came after Russia invaded Ukraine — which isn't a NATO member — in late February. Finland shares a long border with Russia, and Sweden borders Finland.

The move would expand NATO's eastern borders significantly — a result that would most likely anger Russian President Vladimir Putin and his officials, who have long complained about the alliance's eastward expansion and used it as a justification for invading Ukraine.

Russia has also repeatedly threatened consequences against Finland and Sweden if they were to join NATO.

Yet in a jarring reversal this week, Russian officials started downplaying the effect the two nations' joining NATO might have.

Putin said on Monday that Russia would have "no problem with these states" joining the bloc.

Russian Foreign Minister Sergey Lavrov also said on Tuesday that Finland and Sweden had already been "participating in NATO military exercises for many years."

Finland and Sweden's accession, which would boost NATO to 32 members, could take several weeks or months.