Simplify the following text for a Grade 11 native English-speaking person.
Original: After Muhammad 's death , he played a key role in commanding Medinan forces for Abu Bakr in the Ridda wars , the capture of the Sassanid Arab client Kingdom of Al-Hirah , and the defeat of the Sassanid Persian forces during his conquest of the Persian Empire .
Simplified: After Muhammad 's death , he played a key role in commanding Medinan forces for Abu Bakr in the Ridda wars , conquering central Arabia and subduing Arab tribes . He captured the Sassanid Arab client Kingdom of Al-Hirah , and defeated the Sassanid Persian forces during his conquest of Iraq ( Mesopotamia ) .