The ultimate reason for Sparta defeating Athens in the Peloponnesian War was planning, making use of resources, and realizing their own strengths and weaknesses. .
Sparta dominated the Peloponnesian League, which included several small ally states that were easily controlled by Sparta because of how close they were. This dominance over these smaller states rose from Sparta's massive continental army. They were able to have such a massive army because the farming and manual labor was provided by the slaves, which freed up the male citizens to serve in the army. Spartan children were all trained to serve in the military as professional soldiers, where needs of the military and the state come before the needs of the family. .
The Athenian military was structured much differently than Sparta's. Their military grew through voluntary enlisting. They didn't have the ground troop numbers that Sparta had, but their navy was much more dominant. Sparta realized that they couldn't match Athens impressive navy, so Archidamus delayed the battle and worked on extending Sparta's alliances in hopes of expanding their own navy. He realized that he would never have the size of navy that Athens had, so he focused more on his strength, which was his army. He used this strength to take out Athenian allied cities and capture all the land outside the walls of Athens. This tactic froze Athens supply of food and also any additional revenue needed to maintain Athens military. .
Athens strategy was to use its strong navy and pound the coastal lands of Sparta. This strategy worked, but it could only do so much damage. Pericles overall plan was to cluster together inside the Athenian walls, pound Sparta's coastline, and allow Sparta to attack anything they wanted outside the walls. Another hope that Pericles had was that with so many Spartan military gone from their homeland, there was a good possibility that the slaves would revolt against the town, taking advantage of the numbers.