In line to join NATO, Sweden and Finland are facing some 'vulnerable' months

Some have described what is about to transpire between Finland, Sweden and NATO over the next few months as the "marriage vows."

Now that Stockholm has formally acknowledged that it will follow Helsinki in applying for membership in the western military alliance, the long, anxious trip to the altar gets underway with the accession talks.

And so begins what's likely to be several months of tensions for two nations committed to NATO but not quite part of it — and not yet protected by its security guarantees.

Swedish Prime Minister Magdalena Andersson warned that her country would be in a "vulnerable position" during the application period and urged her fellow citizens to brace themselves for Russia's response.

"Russia has said that it will take countermeasures if we join NATO," she said. "We cannot rule out that Sweden will be exposed to, for instance, disinformation and attempts to intimidate and divide us."

The decision by Finland and Sweden to seek NATO membership is historic for many reasons. For Sweden, it means ending two centuries of military non-alignment.

And while there's enthusiasm for expanding NATO in most quarters of the alliance, the marriage itself won't be legal until all 30 existing members ratify the new memberships.

At the moment, the prospect of the two northern nations being stranded at the altar is being downplayed. Foreign Affairs Minister Mélanie Joly said Monday she believes ways can be found to accommodate the concerns raised by Turkey, which already has expressed opposition.

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Accession talks — where NATO officials go over all of the different obligations of membership — are expected to conclude before alliance leaders meet in Madrid at the end of June. During those talks, each nation will be asked a series of questions, including the main one — "Do you agree to uphold Article 5 of the Washington Treaty?"

After Finland and Sweden accept the one-for-all, all-for-one provision in Article 5 — which requires members to come to each other's defence if they're attacked — a number of housekeeping items will follow, such as cost-sharing arrangements and discussions of individual nations' roles in defence planning. There are also legal and security obligations to discuss.

"Some people have called it the marriage vows," a NATO official said recently in a background conversation.

Finnish President Sauli Niinisto said over the weekend that Russian President Vladimir Putin responded "calmly" when he was told that Finland would apply for NATO membership.

Defence and foreign policy experts say no one should read too much into that cordial, somewhat frosty reception.