Complex sentence: Demetrius, son of Antigonus I, besieged Rhodes supposedly in an attempt to break its alliance with Egypt but it was also a piratical enterprise by Demetrius who thought "it was a glorious thing to be a pirate king".
Split into simpler sentences: Demetrius, son of Antigonus I, besieged Rhodes supposedly in an attempt to break its alliance with Egypt. The decision to lay siege to Rhodes was influenced by these fears but it was also a piratical enterprise by Demetrius who thought "it was a glorious thing to be a pirate king".