Simplify the following text for a Grade 8 native English-speaking person.
Original: On the level of international relations , Athens , the strongest city-state in Greece prior to the war 's beginning , was reduced to a state of near-complete subjection , while Sparta became established as the leading power of Greece .
Simplified: Athens , the strongest city-state in Greece before the war started , was reduced to a state of near-complete subjection . Sparta became the leading power of Greece .