I can help.
Ok well basically Athens and Sparta had been allies to help defend Greece from some attack, but then they decided they didn't like each othere because Athens was the commercial and business centre of Greece and Sparta was the military power in Greece. They then had a load of battles until the Persians threatened Greece from the East. Sparta and Athens united together again to Defend Greece against this threat. The Spartans denied help from the Athenians in the battle of Thermopylae as they knew that they would not win the battle and that they should save their men. The Athenians started to build a load of triremes to Defend from sea attacks. They beat the Persians in the Bay of Salamis.
After the threat from Persia had cleared Sparta and Athens went back to war agian despite being allies to defend against Persia. Does this help?