This episode covers the first stage of the Pentekontaetia (i.e. fifty year period) which stretched from 479-435 BCE. Yes, the dating doesn't exactly line up but that doesn't much matter. In this first stage Athens forms its Delian League and within fifteen years of its formation in 478 BCE the Delian League was clearly an Athenian Empire; this, and many other reasons detailed in the episode, made Athens the ire of most of Hellas (and not because of jealousy). This first part of the story of the Pentekontaetia period is instrumental to the beginning of the Peloponnesian War for it brought Athens and Sparta into great contention.