Complex sentence: Leading his confident army north he found the Mamertines again at the Longanus River on the plain of Mylae where he easily defeated them since the Mamertines were not accustomed to large pitched battles and had become reckless after beating Hiero's mercenaries.
Split into simpler sentences: Leading his confident army north he found the Mamertines again at the Longanus River on the plain of Mylae. There he easily defeated them as the Mamertines were not accustomed to large pitched battles and had become reckless after beating Hiero's mercenaries.