 Followed the victory at Qadisiyyah, Muslims captured Ctesiphon, the Sassanid Persian capital city, after two-month siege in March 637 followed by capturing of Tikrit and Mosul.Umar wanted Zagros mountains to be the frontiers between Muslims and Persians and is quoted saying 

Persians kept on raiding Iraq, which resulted in political instability in Iraq, as a result Muslims invaded Ahvaz in 638 which forced powerful Persian commander in chief, Hormuzan, to enter into peace negotiations with Muslims.Hormuzan agreed to accept Muslim suzerainty over Ahvaz province and agreed to rule as Muslim's vassal.