Once the rebellions had been put down, Abu Bakr began a war of conquest. Whether or not he intended a full-out imperial conquest is hard to say; he did, however, set in motion a historical trajectory that in just a few short decades would lead to one of the largest empires in history. Abu Bakr began with Iraq, the richest province of the Sasanian Empire. He sent general Khalid ibn Walid to invade the Sassanian Empire in 633. He thereafter also sent four armies to invade the Roman province of Syria, but the decisive operation was only undertaken when Khalid, after completing the conquest of Iraq, was transferred to the Syrian front in 634.