Simplify the following text for a Grade 9 native English-speaking person.
Original: These battles of Salamis and Plataea thus mark a turning point in the course of the Greco-Persian wars as a whole ; from then onward , the Greek poleis would take the offensive .
Simplified: These battles of Salamis and Plataea mark a turning point in the Greco-Persian wars ; from then on , the Greek cities would take the offensive .