Complex sentence: When Croesus was told by the Pythia that going to war with Cyrus the Great would destroy a great empire, the empire was not Cyrus's but Croesus's.
Split into simpler sentences: When Croesus was told by the Pythia that going to war with Cyrus the Great would destroy a great empire. In the end, the empire destroyed was not Cyrus's but Croesus's.