Simplify the following text for a Grade 10 native English-speaking person.
Original: At the center of this time period is Classical Greece , which flourished during the 5th to 4th centuries BC , at first under Athenian leadership successfully repelling the military threat of Persian invasion .
Simplified: In the middle of this period was Classical Greece , which flourished during the 5th to 4th centuries BC . Athenian leadership successfully repelled the threat of Persian invasion .