Every Easter, Venice's dignitaries board an official boat, sail out into the Adriatic and drop a gold ring into the water, symbolizing by this centuries-old ritual the city's marriage to the sea. For a long time, the union was a splendid and prosperous one. Thanks to its sprawling trade network, Venice became a wealthy imperial power in the 13th century, its institutions later mimicked by the Dutch and English. The city-state's mighty fleets patrolled the Mediterranean, while its merchants haggled at the far reaches of the Silk Road, dispatching the wonders of Asia back to an awed...