Trade. Rome feared losing control of its economy, so they covered up their deficiency by declaring war. By taking Carthage, or at least, destroying it, they would remain the undisputed dominion regardless of what anybody else thought.

Sound familiar? That's because it's a tried and true method of hiding your fuckups on a global scale. So if you see some major power declaring war for no real reason, then know they're stuck deep in the shitter and the war is their struggle out.

Also, I have no idea what I'm talking about. I just remembered Carthage had nice ships and everything else just rolled from there.