It's likely to be the quickest Nato enlargement ever and one that would redraw Europe's security map.
Finnish leaders have announced their belief Finland should join the world's biggest military organisation because of Russia's war in Ukraine. Sweden could follow suit.
Should they apply for membership, the move would have far-reaching ramifications for Northern Europe and transatlantic security.
No doubt it will also anger their large neighbour Russia, which blames, at least in part, its war in Ukraine on Nato's continued expansion closer to its borders.
"You [Russia] caused this. Look in the mirror," Finnish President Sauli Niinisto said this week.
While the country's Parliament still has to weigh in, the announcement means Finland is all but certain to apply — and gain admission — though the process could take months to complete.
Such an expansion of the alliance would leave Russia surrounded by Nato countries in the Baltic Sea and the Arctic and be a stinging setback for Putin, who had hoped to divide and roll back Nato in Europe but is instead seeing the opposite happen.
Nato Secretary-General Jens Stoltenberg has said the alliance would welcome Finland and Sweden with open arms.
Russia's Foreign Ministry warned Moscow "will be forced to take retaliatory steps of military-technical and other characteristics in order to counter the emerging threats to its national security".
Nato's funnelling of weapons and other military support to Ukraine has been critical to Kyiv's surprising success in stymieing the invasion, and the Kremlin warned anew the aid could lead to direct conflict between Nato and Russia.
"There is always a risk of such conflict turning into a full-scale nuclear war, a scenario that will be catastrophic for all," Russia's Security Council deputy head Dmitry Medvedev said.
Here's a look at what Finland and Sweden's membership in the 30-country Nato alliance could mean, as the Nordic partners expect to announce their intention to join within days.
Finland and Sweden
Not neutral like Switzerland, Finland and Sweden traditionally think of themselves as militarily "nonaligned".
But Russia's war in Ukraine and Putin's apparent desire to establish a Moscow-centred "sphere of influence" has shaken their security notions to the core. Just days after he ordered the February 24 invasion, public opinion shifted dramatically.
Support in Finland for Nato membership has hovered around 20-30 per cent for years. It is now more than 70 per cent. The two are Nato's closest partners but maintaining good ties with Russia has been an important part of their foreign policy, particularly for Finland.
Now they hope for security support from Nato states — primarily the United States — in case Moscow retaliates. Britain pledged on Thursday to come to their aid.
The Nordic region
Nato membership for the two, joining regional neighbours Denmark, Norway and Iceland, would formalise their joint security and defence work in ways their Nordic Defence Cooperation pact hasn't. Nordefco, as it's known, focuses on co-operation. Working within Nato means putting forces under joint command.
Accession would tighten the strategic Nordic grip on the Baltic Sea — Russia's maritime point of access to the city of St Petersburg and its Kaliningrad exclave.
Finland and Sweden also join them, along with Iceland, at the heart of the triangle formed with the North Atlantic and maritime areas in the Arctic, to where Russia projects its military might from the northern Kola Peninsula. Integrated Nato military planning will become simpler, making the region easier to defend.
Finland and Sweden are Nato's closest partners. They contribute to the alliance's operations and air policing.
They already meet Nato's membership criteria, on functioning democracies, good neighbourly relations, clear borders and armed forces in lock-step with the allies. After the invasion, they formally boosted information exchanges with Nato and sit in on every meeting on war issues.
Both are modernising their armed forces and investing in new equipment. Finland is purchasing dozens of high-end F-35 warplanes. Sweden has top-quality fighter jets, the Gripen.
Finland says it's already hit Nato's defence spending guideline of 2 per cent of gross domestic product. Sweden is ramping up its military budget and expects to reach the target by 2028. The Nato average was estimated at 1.6 per cent last year.
Putin has demanded Nato stop expanding and in his May 9 speech blamed the West for the war.
But public opinion in Finland and Sweden suggests he has driven them into Nato's arms.
If Finland joins, it would double the length of the alliance's border with Russia, adding a further 1300km for Moscow to defend.
Putin has promised a "military, technical" response if they join. But many troops from Russia's western district near Finland were sent to Ukraine, and those units suffered heavy casualties, Western military officers say.
So far, Moscow is doing nothing obvious to dissuade the two — apart from a couple of incidents where Russian planes entered their airspace.
The Kremlin said its response could depend on how close Nato infrastructure moves toward Russia's borders.
Some at Nato worry the Russians might deploy nuclear weapons or more hypersonic missiles to the Kaliningrad exclave, across the Baltic Sea wedged between allies Poland and Lithuania.