I need a thesis on what lead to the Peloponessian War?
This sounds like a great project. I would say that one of the biggest reason for the war between Athens and Sparta is on account of human pride. This is a reason that is often neglected by scholars, but there is something about human pride that leads to war.
Athens was growing proud and gained an empire. They were mostly a sea power and had a hegemony among many city states and even required tribute to be paid.
Sparta was the other great power of the Greek world. However, their power was not based on the sea, but they were a land power. In fact, Sparta was a truly militaristic city state and their soldiers, hoplites were without peer.
The two powers could have co-existed, but they chose not to. More importantly, the two went to war and continued to fight knowing that all would be lost.
It was only a matter of time that the two cities would go to war. The Greek culture was exceedingly agonistic, that is competetive. This could be a great thesis.