Finland is on the cusp of joining NATO while Sweden is on the verge of following suit.
Here's what you need to know about how the war in Ukraine pushed the two Nordic states closer to the US-backed alliance, and what comes next.
Why haven't Finland and Sweden already joined NATO?
While other Nordic countries like Norway, Denmark and Iceland were original members of the alliance, Sweden and Finland did not join the pact for historic and geopolitical reasons.
Both Finland, which declared independence from Russia in 1917 after the Bolshevik revolution, and Sweden adopted neutral foreign policy stances during the Cold War, refusing to align with the Soviet Union or the United States.
For Finland, this proved more difficult, as it shared a massive border with an authoritarian superpower. To keep the peace, Finns adopted a process some call "Finlandization," in which leaders acceded to Soviet demands from time to time.
Both countries' balancing acts effectively ended with the collapse of the Soviet Union. They joined the European Union together in 1995 and gradually aligned their defense policies with the West, while still avoiding joining NATO outright.
Each country had different reasons for avoiding signing up for NATO pact in tandem with the EU.
For Finland, it was more geopolitical. The threat for Russia is more tangible thanks to the two countries' shared 830-mile border.
"Finland has been the exposed country, and we've been the protected country," former Swedish prime minister Carl Bildt told CNN's Christiane Amanpour in a joint interview alongside former Finnish prime minister Alexander Stubb.
While an independent nation, Sweden's geography puts it in the same "strategic environment" as its liberal democratic neighbors, Bildt said. Finland and Sweden have enjoyed a close partnership for decades, with Stockholm viewing its decision to refrain from joining NATO as a way to help keep the heat off Helsinki. Now, however, Sweden is likely to follow Finland's lead.
"We share the idea that close cooperation will benefit both of us," current Swedish Prime Minister Magdalena Andersson said at a news conference last month alongside her Finnish counterpart, Sanna Marin.
What does NATO membership entail?
The reason most countries join NATO is because of Article 5 of the North Atlantic Treaty, which stipulates that all signatories consider an attack on one an attack against all.
Article 5 has been a cornerstone of the alliance since NATO was founded in 1949 as a counterweight to the Soviet Union.
The point of the treaty, and Article 5 specifically, was to deter the Soviets from attacking liberal democracies that lacked military strength. Article 5 guarantees that the resources of the whole alliance -- including the massive US military -- can be used to protect any single member nation, such as smaller countries who would be defenseless without their allies. Iceland, for example, has no standing army.
Bildt said he doesn't see new big military bases being built in either country should they join NATO. He said that joining the alliance would likely mean more joint military training and planning between Finland, Sweden and NATO's 30 current members. Swedish and Finnish forces could also participate in other NATO operations around the globe, such as those in the Baltic states, where several bases have multinational troops.
"There's going to be preparations for contingencies as part of deterring any adventures that the Russians might be thinking of," Bildt said. "The actual change is going to be fairly limited."