When Russian warplanes staged a mock bombing run on Sweden in 2013, simulated attack on Stockholm, air defences were caught napping. It was the middle of the night and no Swedish planes were scrambled. Instead, two Danish jets belonging to NATO's Baltic mission based in Lithuania, scrambled to shadow the Russians.

Nowadays, Finland, which borders its former ruler Russia, and Sweden are not members of NATO but have increased cooperation with the trans-Atlantic alliance, and the joint declarati...