When Umar ibn al-Khattab became caliph in 634, the Muslim conquests outside the Arabian Peninsula increased, and Al-Ahnaf encouraged his people to participate. When the Persians surrounded a Muslim division led by al-'Alaa ibn al-Hadrami, in Iraq, Umar ordered Utbah ibn Ghazwan to go to their rescue. Utbah sent a force of 12,000, most of them form the Banu Tamim, and among them was al-Ahnaf who played a major role in breaking the siege.