Complex sentence: Leaving the Carthaginians alone at Messana would give them a free hand to deal with Syracuse; after the Syracusans had been defeated, the Carthaginian takeover of Sicily would essentially be complete.
Split into simpler sentences: Leaving the Carthaginians alone at Messana would give them a free hand to deal with Syracuse. After the Syracusans had been defeated, the Carthaginian takeover of Sicily would essentially be complete.