Sweden and Finland announce they'll apply for NATO, defying Russia's threats and ending their historic neutrality

• Finland and Sweden have both announced that they will apply to join NATO.
• This is a major and historic development that is sure to enrage Putin.

Finland and Sweden have announced their intention to join NATO, and the alliance has signaled it will welcome both Scandinavian democracies with open arms as the alliance spreads north to deter Russian aggression.

Finnish Prime Minister Sanna Marin and President Sauli Niinisto on Sunday confirmed that Finland would apply for NATO. "This is a historic day. A new era begins," Niinisto told reporters, the Associated Press reported.

Similarly, Swedish Prime Minister Magdalena Andersson on Monday said her country would inform NATO that "we want to become a member of the alliance," per the The Guardian.

"There is a broad majority in Sweden's parliament for Sweden to join NATO," Andersson said, adding. "This is the best thing for Sweden's security."

Any decision on NATO enlargement requires unanimous agreement from all current members. Turkish President Recep Tayyip Erdogan last week signaled that he might move to block Finland and Sweden from joining, citing their alleged support for Kurdish militants that Ankara considers to be terrorists. But NATO Secretary General Jens Stoltenberg expressed confidence that Turkey's concerns will be addressed and Ankara won't derail Finland and Sweden's ambitions of joining the alliance.

"Turkey has made it clear: Their intention is not to block membership," Stoltenberg told reporters Sunday, per the Washington Post. "Therefore, I am confident we'll be able to address the concerns that Turkey has expressed in a way that doesn't delay the accession process."

'Everything changed when Russia invaded Ukraine'

The historic development puts both countries on the brink of a dramatic break from a longstanding tradition of neutrality, and stands as one of the starkest examples of how the Ukraine war has backfired on Russian President Vladimir Putin.

The Russian leader routinely complained of NATO's long-standing eastward expansion in the lead-up to the 2022 invasion, warning the alliance against accepting new members — particularly Ukraine and Georgia. Adding Sweden and Finland to NATO is sure to enrage Moscow, which threatened both with "serious military and political" retaliation over such a move.

Finland shares an 830-mile border with Russia, and has gone to war with it in the past. The Finns and Russians fought during World War II — a conflict known as the Winter War. Though Finland put up a fierce resistance against the Red Army, it was ultimately defeated by the Soviet Union and forced to agree to a peace agreement that saw it lose roughly 10% of its territory.

In an effort to avoid similarly devastating conflicts, Finland remained militarily non-aligned during the Cold War.

Finland and the Soviet Union in 1948 signed a treaty that ensured Helsinki would stay neutral as the Soviets pledged not to invade again. Though Finland maintained its sovereignty during this era, Moscow had immense influence over Finnish politics and its statehood was severely limited.

Political scientists would eventually employ the term "Finlandization" to describe the restricted form of statehood Finland endured during this period. In diplomatic discussions that occurred in the lead-up to the Russian invasion of Ukraine in late February, some suggested that "Finlandization" was a potential model for Kyiv to accept in order to avert a broader conflict.

Sweden also remained neutral during the Cold War. In fact, Sweden has not been a combatant in any war for over a century. However, Sweden's neutrality has long been a matter of debate — particularly in relation to World War II. The Swedes permitted the Nazi army to transit its country and years later, as the tide was turning against the Germans, allowed the Allies to use some air bases.

After the collapse of the Soviet Union, both countries became NATO partner countries and eventually joined the European Union. But until Russia's unprovoked invasion of Ukraine, neither Sweden nor Finland appeared particularly eager to pursue full NATO membership. At a joint press conference with her Swedish counterpart in Stockholm in mid-April, the Finnish prime minister said, "Everything changed when Russia invaded Ukraine."

Indeed, Russia's war in Ukraine prompted swift, dramatic changes in tone from both Sweden and Finland on the subject of NATO membership. After the invasion, polling in both countries showed rapidly growing levels of public support for joining the alliance. Finland's president last week told Russia to "look at the mirror" if the Finnish government ultimately decided to pursue NATO membership.

Russia has threatened to retaliate if Finland and Sweden join NATO

NATO membership will require Sweden and Finland to honor Article 5, the principle of collective defense enshrined in the alliance's founding treaty. The 30-member alliance considers an attack on one member an attack on all. Article 5 has only been invoked once — following the 9/11 terror attacks in the US. But NATO has conducted military operations in past conflicts — such as in Kosovo and Libya — without being attacked first.

The alliance has expressed a strong desire to avoid a direct confrontation with nuclear-armed Russia, and has maintained that it will not send troops into Ukraine to help it fight Russian forces. Along these lines, NATO has also declined requests from Kyiv to set up a no-fly zone in Ukraine, as it would require alliance countries to shoot down Russian warplanes and effectively amount to a declaration of war. Though they've avoided direct involvement in the war, NATO countries have been pouring military assistance — including lethal aid — into Ukraine.

Even before applying to NATO, Finland and Sweden broke from their traditions of neutrality to send weapons and other military aid to Ukraine after Russia invaded.

Both Finland and Sweden have significant military assets to offer NATO.

"The Finnish Army remains pretty exceptional in Europe," Finnish defense analyst Stefan Forss told Foreign Policy in April. "We train large cohorts of conscripts each year, and what they're expected to master is the defense against a major Russian attack. That has been the focus of their training even during the past few decades, when virtually every other country switched to crisis management using small units. At our defense college, the general staff officers are still trained to lead brigades and corps."

And amid concerns over the growing threat presented by Russia in recent years, Sweden has bolstered its navy — particularly in the Baltics.

Though Russia has threatened to retaliate against both countries for pursuing NATO membership, it's unclear what its response might look like. Russia has been politically and economically isolated by the Ukraine war, and has suffered staggering troop losses. Under these circumstances, Moscow's capacity to take military action against countries that have the backing of an alliance whose members include several nuclear powers is seemingly quite limited.

Finland's pro-NATO former prime minister told Insider in April that he expected Russia's retaliation to come in the form of a cyberattack, but said Finland is prepared.

Staff writer Sinéad Baker contributed reporting.