When Umar took charge of the caliphate, the Muslim armies were fighting against the Persians in Iraq and the Romans in Syria. The army in Syria was under the command of Khalid bin al-Walid, the favorite general of Abu Bakr. Umar's first act as khalifa was to dismiss him from all his commands, and to appoint Abu Obaida bin al-Jarrah as the supreme commander of the Muslim forces in Syria.
Shibli says that Umar had, for a long time, nursed a secret hatred of Khalid because of the latter's excesses. Umar had indeed dismissed Khalid because of his excesses but it appears that personal rancor was also at work. He was jealous of Khalid's fame and popularity. If he disliked Khalid's transgressions, he ought to have formally indicted him, and should have ordered full investigation of his crimes in murdering Malik ibn Nuweira and in appropriating his widow. If Khalid had been proven guilty, then Umar ought to have passed sentence on him according to the Islamic law. But there was no indictment and no investigation. Khalid was summarily dismissed and he died in poverty and obscurity in 21 A.H.
Umar's caliphate is notable for its many conquests. His generals conquered Iraq, Iran, Azerbaijan, Kirman, Seistan, Khurasan, Syria, Jordan, Palestine and Egypt, and incorporated them into the empire of the Muslims. All of these were permanent conquests. The Romans lost Syria, Palestine and Egypt for ever; and in Persia, the Sassani empire ceased to exist.
Among other events of the caliphate of Umar, were the first outbreak of plague in Syria in 18 A.H., and a famine in Hijaz in the same year. Between them, the plague and the famine killed more than 25,000 people (Suyuti and Abul Fida).