But as we saw in the last video, as soon as the Persians are dealt with, tensions start to rise between Athens and Sparta and their various allies. Sparta gets worried that the Athenian navy is looking maybe a little bit too strong. They were decisively important in the second Persian invasion. 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.