Khalid bin Walid who was a cousin of the mother of Umar was the hero of the apostasy wars conducted during the caliphate of Abu Bakr. While Umar appreciated Khalid's skill as a General he was critical of Khalid's moral conduct.

Having defeated Taleaha at the battle of Buzakha, and reduced the tribes in the north Khalid bin Walid decided to march against the Bani Tamim who lived on a plateau bordering on the Persian Gulf. The Bani Tamim had accepted Islam during the life time of the Holy Prophet. After the death of the Holy Prophet when the wave of apostasy spread over the Arabian peninsula, the Bani Tamim were also affected. The tribe came to be divided into two sections. One section remained faithful to Islam while the other section apostatised.

When Khalid gave his army the order to march to Bataha the headquarter of the Bani Tamim a section of the army objected to the order on the ground that the Caliph had not sanctioned any action against the Bani Tamim. The objection was overruled by Khalid.

The orders of Abu Bakr were that if any tribe professed faith in Islam, no action was to be taken against it. If a tribe did not profess faith in Islam, it was to be invited to accept Islam, and operations were to be undertaken against it only in the event of refusal. The strategy laid down was that if on reaching a settlement the residents pronounced Adhan, it was to be understood that the people were Muslims. In the absence of such response it was to be presumed that the people were hostile to Islam.

Before the Muslim army reached Bataha, delegation from Bani Tamim waited on Khalid. They brought with them the necessary amount of the tax payable to the Muslims. Khalid took the amount, but continued his advance to Bataha. When the forces of Khalid reached Bataha there were no forces of the Bani Tamim to oppose the Muslims. The position was confused. Malik the chief of Bani Tamim neither came forward to offer his submission, nor did he come forward to oppose the Muslims.

Khalid directed his soldiers to forage in the neighbourhood. Malik and his wife Laila were taken captive and brought before Khalid. Malik's wife Laila was known far and wide for her breath-taking beauty. Her long glossy hair flowed upto her knees. She had gorgeous legs and she carried herself with peculiar grace and charm. In Khalid's camp Malik was killed and Khalid married Laila.

This led to considerable scandal. In some quarters it was held that Malik was indeed a Muslim and that he had been killed because Khalid coveted his beautiful wife Laila. Some of the Ansars in the army of Khalid led by Abu Qatadah withdrew from the army of Khalid. Abu Qatadah along with Mutamim the brother of the late Malik set out for Madina to lodge a complaint against Khalid. Mutamim was a distinguished poet, and he composed a heart rending elegy mourning the death of his brother. The elegy became very popular in Maclina, and those who listened to it felt sympathy for Malik.

Khalid was summoned to Madina and put to explanation. Khalid's defense was that if according to the Holy Prophet he was the 'Sword of Allah' how could such sword fall against the neck of a Muslim? Umar was highly critical of the conduct of Khalid and held that he was guilty of murdering a Muslim to marry his beautiful wife. As the false prophet Musailma had defeated the Muslims twice, and Khalid's services were required to defeat Musailma, Abu Bakr took a lenient view, and decided that blood money should be paid out of the Baitul Mal to the heirs of Malik. Umar did not feel happy over the decision.

Khalid fought against Musailma in what came to be known as the battle of Yamama. It was a great trial of strength and though the Muslims won a victory, this was achieved at a heavy cost. Over 14,000 followers of Musailma died in the battle. Twelve hundred Muslims fell as martyrs in the battle and though the number was very much less than the number of dead of Banu Hanifa, the tribe of Musailma, yet the Musllm loss was quite heavy. Among the martyrs was Zaid the brother of Umar. Umar felt much grieved at the death of his brother. He used to say "Whenever the breeze blows from Yamama it brings to me the fragrance of Zaid".

Terms with the Banu Hanifa were negotiated by Khalid with Maja'a. Maja'a had a beautiful daughter and one of the terms stipulated by Khalid was that Maja'a should marry his daughter to him. Maja'a hesitated but Khalid forced him to marry his daughter to him the same day that the treaty was signed. Umar was critical of the conduct of Khalid, and complained to Abu Bakr. Abu Bakr wrote a letter to Khalid reprimanding him in the following terms:

"O son of the mother of Khalid. What has gone wrong with you? You are out to wed women when the land around your camp is still drenched with the blood of over a thousand martyrs."

In Iraq, in the battle of Daumatul Jandal fought in 633 AD, Khalid married the beautiful daughter of the chief Judi bin Rabee'a. Umar spoke critically of this marriage to Abu Bakr. Abu Bakr disposed of the matter with the remarks:

"Khalid has a soft corner in his heart for beautiful women. He is the victor, and he may well have Bint Judi as his prize, if that is his pleasure."

At the battle of Muzayyah in Iraq fought under the command of Khalid two Muslims, Abdullah and Labid were killed. Khalid was criticised for killing two Muslims. Umar was very bitter and pressed for action against Khalid, Abu Bakr again took a lenient view. He held that such things were likely to occur when Muslims chose to live in the midst of non-Muslims against whom military operations were undertaken. Abu Bakr paid blood money out of the Baitul Mal to the heirs of the two persons who had been killed.