Complex sentence: War compelled Plato to leave Sicily during his second visit there; before permitting him to leave, however, Dionysius had extracted a promise that he would return when hostilities ceased and Plato had agreed on the condition that Dion be recalled from exile.
Split into simpler sentences: War compelled Plato to leave Sicily during his second visit there. Before permitting him to leave, however, Dionysius had extracted a promise that he would return when hostilities ceased and Plato had agreed on the condition that Dion be recalled from exile.