The imperial province of Venetia and Istria at the head of the Adriatic Sea was as old as the Roman Empire. It had been the tenth of the eleven regions of Italy marked out by the first emperor Augustus. Encircled by the foothills of the Alps on the north and itself encircling the sea, its southern border in Italy was fixed by the river Po, to the south of which, on the Adriatic shore, lay the imperial naval base of Classis or Ravenna. Its main cities were Padua, Aquileia and between them Opitergium or Oderzo. It was a mainland province whose inhabitants saw no promise in the muddy islands and lagoons on to which they looked. Fish could be caught there and salt could be collected. But no one could have foreseen that the islands would have a greater future than the continent. It was said in later years that the Veneti or Venetici were so called both in Greek and in Latin because they were a praiseworthy people. It was a scholarly but fanciful etymology. For in truth the original Veneti were neither Greek nor Latin but Illyrian.

By the fourth century AD, when Constantine the Great became the first Christian Emperor of the Romans, the christianisation of Venetia had already been assured by the establishment of a bishopric at Aquileia. The province lay much nearer to Rome than to the new capital of the civilised world set up by Constantine on the site of the ancient Byzantium. But it was still a mainland province of the undivided Roman Empire.