With the first two, communication could be had only by sea.
From Spain, where his firmest support was found, he could be reached by both land and sea, unless an enemy barred the passage; but the sea route was the shorter and easier.
In the first years of the war, Rome, by her sea power, controlled absolutely the basin between Italy, Sicily, and Spain, known as the Tyrrhenian and Sardinian Seas.

The sea-coast from the Ebro to the Tiber was mostly friendly to her.

In the fourth year, after the battle of Cannae, Syracuse forsook the Roman alliance, the revolt spread through Sicily, and Macedonia also entered into an offensive league with Hannibal.

These changes extended the necessary operations of the Roman fleet, and taxed its strength.

What disposition was made of it, and how did it thereafter influence the struggle?
The indications are clear that Rome at no time ceased to control the Tyrrhenian Sea, for her squadrons passed unmolested from Italy to Spain.