By extending their arguments to address entire nations, some social Darwinists justified imperialism on the basis that the imperial powers were naturally superior and their control over other nations was in the best interest of human evolution.
The Persian wars were fought against a foreign invader.
The Peloponnesian war was fought among Greeks.
They were fast moving so they took over lots of land then united them by faith. It also helped that they controlled the Silk Road, the biggest trade network between Europe and Asia.
Answer:
because it became less economically viable to push the borders further, and because of the Persians in the east.
