Sweden has undergone a seismic shift in military strategy after decades of demilitarization in order to prepare to repel Russian invaders. As a shadow war heats up in the Baltic Sea over critical subsea cables, NATO’s newest member is on the front lines with a strategic island called Gotland. Troops here are training in drills to repel invaders, readying civilian bomb shelters built during the Cold War and more. WSJ’s Sune Engel Rasmussen visits Gotland to understand why it’s undergoing a