And they build this Delian League, whose purpose is to go on the offensive against the Persians, but more and more so, it's starting to look like an Athenian empire. And a lot of this, you can imagine, it's like a game of risk. The more you take over, the more resources you have, the more that you can build more ships and have more soldiers so that you can take on even more. So as Sparta and Athens are starting to become more and more rivals, Sparta's getting insecure about Athens' influence and their resources and their military power. And we even saw it in the last video. You have an earthquake in Sparta, potentially right around the same time that Sparta was planning an invasion of Athens, leaving the Spartans vulnerable. There's a helot uprising, these Spartan slaves.