Context : Umar, also spelled Omar (Arabic: عمر بن الخطاب‎, translit. ʿUmar ibn Al-Khattāb, Umar Son of Al-Khattab, born c.583 CE -- died 3 November 644 CE), was one of the most powerful and influential Muslim caliphs in history. 
Subject : Umar 
Relation : conflict
What war was Umar in?