It is a clement, early summer evening in Visby, and German, Finnish and Danish voices mingle in the air with eurotrash music coming from a bar on Stora Torget, literally the “Great Square”. Some teenagers are walking among the nearby ruins of St Karin’s church. “It’s like Gotland’s version of the Tower of Pisa,” one of them says. “Don’t be so stupid,” his friend laughs.

Then there is a crack in the skies above, and a Swedish Gripen fighter jet zooms past, drawing scant attention from the people below cradling glasses of beer and wine. In one corner of the square sits a bar called Ryska G?rden, a former Russian trading post with the remnants of a Russian church beneath it. “We’re here on holiday,” says a couple from northern Sweden. “Not to think about war.”

But that is why I’ve come to Gotland, the idyllic 1,200-square-mile Swedish island in the middle of the Baltic Sea. On the one hand, it is one of the tourist pearls of northern Europe with dozens of perfect, white beaches as well as the medieval walled town of Visby. But it is also, in the words of one of the soldiers I meet here, an “unsinkable aircraft carrier in the middle of the Baltic”.