Complex sentence: Cosa was an important colony and military outpost in Etruria, and Pisae was the only port between Genua and Rome; as such it served as an important naval base for the Romans in their wars against the Ligurians, Guals and Carthaginians.
Split into simpler sentences: Cosa was an important colony and military outpost in Etruria, and Pisae was the only port between Genua and Rome. Consequently, it was an important naval base for the Romans in their wars against the Ligurians, Guals and Carthaginians.