Complex sentence: Elected "king" by 398 BC, Himilco led the Carthaginian effort against Dionysius after 398 BC, and although initially successful, suffered a reverse at Syracuse in 396 BC when his forces were decimated by the plague and then defeated by Dionysius.
Split into simpler sentences: Elected "king" around 398 BC, Himilco then led the Carthaginian effort against Dionysius from that date. Although initially successful, Himilco suffered a reverse at Syracuse in 396 BC when his forces were decimated by the plague and then defeated by Dionysius.