The security policies of Sweden and Finland have changed more in the past six weeks than they have over the past six decades. In much of what they do, the two countries come as a pair. Their defense cooperation has deepened since Russia’s invasion of Crimea. Now, the two appear poised to break with their long history of non-alignment.

Their applications to join the North Atlantic Treaty could come in time for NATO’s summit in Madrid in late June. At a press conference in Stockholm this week, the Swedish and Finnish prime ministers  — Magdalena Andersson and Sarin Marin — stopped short of announcing that they want their countries to join the Western military alliance.

“This is an important time in history,” Ms. Andersson, speaking alongside Ms. Marin, said. “The security landscape has completely changed.”

The gravity of such a declaration cannot be overstated. Sweden has been neutral since the time of the Napoleonic wars. Its non-alignment has long been a source of pride. The Finnish case is more complicated, not least owing to its geography. The country became independent from Russia in 1917 and after its war with the Soviet Union between 1939 and 1940 was ostensibly forced into isolation. 

Stockholm and Helsinki have therefore historically sought a third way between Russia and the West. That history could now be abandoned, pushing Europe closer to its Lyapunov horizon — past the point of predictability.

For it is not clear, exactly, what either country would bring to the alliance — an extended border, that is clear, but what of troops, defense spending, and strategy? Sweden managed to stay neutral during World War II and has not fought a war since 1814. Also unclear is how Western security could be altered as a result.

Until recently, the leaders of Sweden and Finland also seemed to be unsure. Sweden’s defense minister, Peter Hultqvist, has repeatedly ruled out NATO membership. Last year, he said that membership was never going to be on the cards as long as he remains in his job. 

He still holds his job. Just last month, Ms. Andersson also dismissed the project and said an application to join NATO would have a destabilizing effect on Nordic security. She seems to have changed her mind. 

Yet the fear of stoking tensions lingers. Just a few days before Ms. Andersson’s comments about NATO membership, four Russian jet fighters violated Swedish airspace outside the Swedish island of Gotland in the Baltic Sea.

An eventual Russian move on the Baltics would likely be preceded by an occupation of Gotland, which would serve to close the air space to NATO forces. 

In response to Sweden and Finland’s policy shifts, Dmitry Medvedev, deputy head of Russia’s Security Council, has also warned that Moscow would strengthen its defenses, including possible nuclear deployments in the region.

Nevermind that Russian nuclear-armed Iskander cruise missiles have been stationed in Kaliningrad since 2016. Could a further build-up be in the cards? 

The Swedish fear of Russian retaliation is not fiction. Yet Sweden’s  desire for NATO membership is not gospel either. Ms. Andersson’s party, the Social Democrats, yearns for neutrality. Its members combine habitual anti-Americanism with a desire to restore Swedish foreign policy in the image of Olof Palme.

Palme was an anti-nuclear and anti-imperialist champion of the Third World who governed in the 1970s and 1980s. Together with Germany’s Willy Brandt and Austria’s Bruno Kreisky, Palme led the group of new internationalists who sought a middle way between the Soviets and the West.

The Finnish debate also carries echoes from the past — mostly from those with anti-American chips on their shoulders. Yet Finland is ostensibly more convinced of its bid for membership. 

In the fog of war, Sweden and Finland’s accession to NATO seems to be the logical step for the national defense of both nations — and it may well be. Yet, as Lyapunov’s horizon cautions, decisions of great magnitude should be made gingerly, with a view to second-  and third-order consequences, long-term time horizons, and desired national identities. At least for now, Stockholm and Helsinki appear intent on skipping this process.