The Exarchate of Ravenna was an ad hoc organisation of the Byzantine dominions in Italy, established c.584, in the face of the Lombard invasion and gradual conquest of much of the Italian peninsula.
Venice was an independent state for almost a millennium, but it might note have been. It all hinged on a single battle fought in the Venetian lagoon in 810.
Recently discovered frescoes in the church of Torcello, former seat of the bishop, attest the close connections early Venice had to the Caroligian empire.