After successful campaigns against the Sassanids and the ensuing conquest of Iraq, Khalid established his stronghold in Iraq. While engaged with Sassanid forces, he also confronted the Ghassanids, Arab clients of the Byzantines. Medina soon recruited tribal contingents from all over the Arabian peninsula. Only those who had rebelled during the Ridda wars were excluded from the summons and remained excluded from Rashidun armies until 636, when Caliph Umar fell short of manpower for the Battle of Yarmouk and the Battle of al-Qādisiyyah. The tradition of raising armies from tribal contingents remained in use until 636, when Caliph Umar organised the army as a state department.