Venice and the Sack of Constantinople
Venice was a republic of merchants built on mud, with no farmland and no army worth the name, that made itself rich as the hinge between Europe and the East. In 1204 the Fourth Crusade, deep in debt to Venice for its ships, was redirected — with Venetian encouragement — to Constantinople, the greatest Christian city in the world, which the crusaders sacked and stripped. Byzantium never recovered. A crusade launched to fight Islam destroyed the eastern Christian empire instead, and its financier took the trade routes.
The path or medium of exchange moves, and a place or power rises or declines because it sits on or off the new route.