The Greeks had an advantage over their attackers during the Persian Wars. They were fighting to protect their homes and their way of life. The Greeks proved they were better soldiers than the Persians. They were better trained and better skilled.
After the war the Greek city states became stronger by joining forces with other city states. The Greek city states divided into 2 different leagues.
The Greek city states now felt a lot more confident and freedom now that the Persian threat wass over. Athens now became the leader of an alliance called the Delian League.