Complex sentence: Polycrates sent 40 triremes to the Persians, and expected the politically dangerous crew to be executed by Cambyses, but halfway through the trip they realized what was happening and turned the ships around.
Split into simpler sentences: Polycrates sent 40 triremes to the Persians, and expected the politically dangerous crew to be executed by Cambyses. But half way through the trip they realized what was happening and turned the ships around.