The Fourth Crusade was initiated and manipulated by the Doge of Venice, and Venice would've likely been ruled by the Byzantines in the 1200s if the Muslims had not conquered Egypt and Syria. Actually, had the Byzantines consolidated their hold in Italy again, Venice probably wouldn't have amounted to much at all. It was founded by refugees fleeing the Lombards in 568 and really didn't amount to much in the 7th century. Its value was its geographic defenses and if reasserted Byzantine rule had mitigated the endemic threat of marauding thugs many of its later residents would've likely stayed on firmer ground..
Read City of Fallen Angels. It isn't as good as the guy's Savannah book, but it is good. I finally got a handle of what Doge meant. Before that, the only context I had was the Doge of Downey, California, home of a division of Bechtel.