d. Persia prevented Sparta and Athens from uniting.
The Persian conflicts were influenced the Greek city-states because they formed under the administration of Athens and were to nevermore again penetrate the Persian Armies.
One significant cause that classical Greece was controlled by small city-states and autonomous towns, rather than by one all-powerful leader. An ultimate purpose following the advancement of city-states was the Greek nobility, who worked to block any perpetual governments from working.