During World War II and the German invasion of Greece (6/4/1941), the few defenders of the forts of the so-called SILK LINE, on the Greek-Bulgarian border, bravely resisted the Nazi hordes. Istibei on the top of Beles, fell only when the enemy threw poisonous gases into the underground galleries. Rupel, which was literally a breakwater for the invaders, surrendered after three days of attacks and after Thessaloniki had previously been occupied. The German Colonel who received Rupel, congratulated Commander Duratsos and expressed his admiration and appreciation for the resistance and heroism of the Greek defenders of the fort. In the two forts, which are open to the general public, you can walk through the labyrinthine underground galleries with machine guns and observation posts, and learn about the history in the small museums accompanied by military guides. The simple monuments of the forts are inscribed with the names of the dead who fell heroically fighting the invaders. Every year on the first Sunday of May, anniversary events are held in the forts.