Simplify the following text for a Grade 10 native English-speaking person.
Original: Later in the Classical period , the leagues would become fewer and larger , be dominated by one city ( particularly Athens , Sparta and Thebes ) ; and often poleis would be compelled to join under threat of war ( or as part of a peace treaty ) .
Simplified: Later in the Classical period , the leagues would become fewer and larger , be dominated by one city ( particularly Athens , Sparta and Thebes ) .