Abu Bakr was 63 years of age when he caught fever. He tried performing his duties but over time he was seceding more and more responsibilities on Umar due to his illness.
When the fever became too sever, Abu Bakr called on his chief advisors, Shura. All eyes were already on Umar, only few complained and too about his temper.
Abu Bakr replied, “he is severe, for he thinks I am lenient. But when the responsibilities fall on him, he will be softened. He is, indeed, the best amongst you.”
After this, Abu Bakr summoned all the people outside the Madinah mosque and told he has chosen Umar as his successor who isn’t his relative but the best amongst them. The people replied that they would obey him.
Umar was 51-years old when he was made a Caliph. He was the son of Al-Khattab, a scion of Bani Adi. Umar was a notable figure in pre-Islamic Arabia. He was known for his bravery and ardent Arabian nationalism. This is why he opposed Islam so vigorously in start.
He would go on to conquer Iraq, Persia, and Egypt.