Venice didn’t appear out of nothing – there was something before which led to the Venice we know as the Serenissima.
The earliest written source related to the inhabitants of the Venetian lagoons, is a letter from Cassiodorus, written in 537.
Venetia existed within the Exarchate of Ravenna, as part of the Byzantine Empire, and what happened in the Exarchate mattered to Venetia.
The name Venice is from the ancient tribe of the Veneti, whose lands became a Roman province, with the name of “Venetia et Histria”