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.