An international cat-and-mouse game played out in the waters of Stockholm a few months ago.
The "mouse" was a foreign submarine — Russia is the main suspect — that got away.
And as Russia's military becomes more aggressive, European leaders fear they do not have the military power to deal with this new threat.
Take Sweden, for instance. Its days of military might are long gone.
The numbers tell the story, says Karlis Neretnieks, who used to run Sweden's National Defense College and has had a long career in the military.