Complex sentence: During Abu Bakr's Era Khalid ibn al-Walid who conquered Iraq was sent to Syrian front to command the Islamic armies there, as soon as Khalid left Iraq with his half army of 9000 soldiers Persians decided to take back their lost territory.
Split into simpler sentences: During Abu Bakr's era, Khalid ibn al-Walid who had conquered Iraq, was sent to the Syrian front to command the Islamic armies there. As soon as Khalid had left Iraq with half his army of 9000 soldiers, the Persians decided to take back their lost territory.