From Teutonic fortresses to modern missile garrisons, the contested geography of the Baltic has made Königsberg, now Kaliningrad, a recurring European flashpoint. Russian naval and missile forces make Kaliningrad a strategic hinge on NATO’s eastern flank. In 1736, Swiss polymath and mathematician Leonhard Euler sat down to solve a quirky civic obsession that Königsberg was renowned for: how to stroll across all seven bridges of the historic capital of the East Prussian city e