Varied causes for Khalid's dismissal from the supreme command are cited by the Islamic tradition. Among these were his independent decision-making and minimal coordination with the leadership in Medina; older allegations of moral misconduct, including his execution of Malik ibn Nuwayra and subsequent marriage to Malik's widow Umm Tamim bint al-Minhal; accusations of generous distribution of booty to members of the tribal nobility to the detriment of eligible early Muslim converts; the personal feud between Khalid and Umar; and Umar's uneasiness over Khalid's heroic reputation among the Muslims, which he feared could develop into a personality cult. The modern historians De Goeje, William Muir and Andreas Stratos viewed Umar's enmity with Khalid as a contributing cause of Khalid's dismissal, while Shaban acknowledges the enmity but asserts it had no bearing on the caliph's decision. De Goeje dismisses Khalid's extravagant grants to the tribal nobility, a common practice among the early Muslim leaders including Muhammad, as a cause for his sacking. Muir, C. H. Becker, Stratos and Philip K. Hitti have proposed that Khalid was ultimately dismissed because the Muslim gains in Syria in the aftermath of Yarmouk required the replacement of a military commander at the helm with a capable administrator such as Abu Ubayda. Athamina doubts all the aforementioned reasons, arguing Umar's "motive for Khalid's dismissal must have been vital" at a time when large parts of Syria remained under Byzantine control after Yarmouk and Heraclius had not abandoned the province, and "Abū ʿUbayda, with all his military limitations, could not have provided a worthy replacement for Khālid's incomparable talents". According to Athamina, Medina's lack of a regular standing army, the need to redeploy fighters to other fronts and the Byzantine threat to Muslim gains in Syria required the establishment of a defense structure based on the older-established Arab tribes in Syria which had traditionally served as confederates of Byzantium. After Medina's entreaties to the Ghassanids were rebuffed, relations were established with the Banu Kalb, Judham and Lakhm. These tribes likely considered the large numbers of outside Arab tribesmen in Khalid's army as a threat to their political and economic power. Khalid's initial force of 500–800 men had swelled to as high as 10,000 as a result of tribesmen joining his army's ranks from the Iraqi front or Arabia and as high as 30,000–40,000 factoring in their families.