Most wars begin because one nation is scared of a nation getting too big or too strong. The Peloponnesian war began because of Sparta’s fear of the Athens getting too strong. Sparta eventually won the war and became the strongest nation. There were still some wars now and then competing for the strongest nation, but in the end everyone just didn’t care and the war ended.
The Persian wars started when King Darius of Persia wanted to expand his nation. They started by invading Greece. The Persian’s lost many man in later wars and it seemed as if the Persians would win the war, but soon enough Greece won. We can only guess what would of happened if the Persian’s won.