Complex sentence: Umar appointed Saad ibn Abi Waqqas as commander for the campaign in Iraq who left Madinah with his army in May 636 and camped at Qadisiyyah in June.
Split into simpler sentences: Umar appointed Saad ibn Abi Waqqas as commander for the campaign in Mesopotamia. Saad left Medina with his army in May 636 and arrived at Qadisiyyah in June.