Simplify the following text for a Grade 9 native English-speaking person.
Original: Aware that his force was being outflanked , Leonidas dismissed the bulk of the Greek army , and remained to guard the rear with 300 Spartans , 700 Thespians , 400 Thebans and perhaps a few hundred others , the vast majority of whom were killed .
Simplified: Aware that his force was being outflanked , Leonidas dismissed the bulk of the Greek army , and remained to guard the rear with 300 Spartans , 700 Thespians ( who refused to leave ) , 400 Thebans and perhaps a few hundred others . Most of them were killed .