On assuming office as Caliph, the first official order that Umar passed as the Caliph was to depose Khalid from the chief command of the Muslim forces in Syria.
Umar addressed his order to Abu Ubaida as follows:
"I urge upon you the fear of Allah Who lives eternally while everything else perishes, Who has guided us away from wrong doing and taken us out of darkness into light. I appoint you Commander of the army instead of Khalid bin Waleed. So take charge from him as is your duty.
Send not the Muslims to their destruction for the sake of plunder; and place not the Muslims in a camp without reconnoitring it and knowing what is there.
Send not expeditions except in properly organised units. And beware of taking any steps which may lead to the annihilation of the Muslims.
Allah has tried me with you, and tried you with me. Guard against the temptations of this world lest they destroy you as they have destroyed others before you; and you have seen how they fell."
The Caliph instructed the messenger to carry the letter to Syria and hand it over personally to Abu Ubaida.
The messenger arrived with the letter at Damascus in the first week of September 634, and handed over the letter to Abu Ubaida. Abu Ubaida read the letter, but he felt that with the siege of Damascus in progress, that was not the opportune time for making a change in the command. He kept the letter with him as a closely guarded secret, and proceeded to act as if no orders had been received from Madina.
When Damascus fell, the pact with the Byzantines was signed by Khalid, Abu Ubaida had offered amnesty to the Byzantines over the head of Khalid, but even when Khalid felt annoyed, Abu Ubaida merely argued in conciliatory terms, and did not drop a hint that he had indeed acted with due authority.
After Khalid had returned from his campaign of the 'Meadow of the Brocade', and written a report addressed to Abu Bakr, Abu Ubaida could no longer keep the letter of the Caliph as a secret. Reluctantly he handed over the letter of Umar to Khalid. Khalid read the letter, and was shocked at its contents.
Turning to Abu Ubaida, Khalid said, "This letter must have reached you about a month ago: why did you conceal it from me?" Abu Ubaida said that he did not wish to sheaken his authority while he was engaged with the enemy.
Khalid gave the charge of the command to Abu Ubaida. The options before Khalid were to retire, or to seek transfer to some other front Khalid did not avail of these options, and he chose to serve in Syria under the command of Abu Uhaida. Khalid said that he was fighting in the name of Allah and it made no difference to him whether he held the command or fought under the command of someone else.