Complex sentence: In short, the entire Athenian fleet must be dispatched to Artemisium; in order to do this, every able-bodied Athenian male would be required to man the ships; and this in turn meant that the Athenians must prepare to abandon Athens.
Split into simpler sentences: In short, the entire Athenian fleet must be dispatched to Artemisium. This in turn meant that the Athenians must prepare to abandon Athens.