In addition to his universal history, Ibn al-Athir also wrote a rather more partisan dynastic history of the Zangids, al-Ta’rikh al-Bahir fi’l-Dawla al-Atabakiyya (The Dazzling History of the Atabeg State). The following extracts from this work give some sense of the tensions that arose between Nur al-Din and Saladin after the destruction of the Fatimid caliphate of Egypt.
Nur al-Din sent to Saladin, ordering him to gather the armies of Egypt and take them to the land of the Franks, and to descend upon Kerak and besiege it, so that Nur al-Din could also gather his armies and travel there, and they could join together to fight the Franks and take possession of their territory. So Saladin set out from Cairo on 20 Muharram [AH 567 (22 September 1171)], and he wrote to Nur al-Din to inform him that his departure had not been delayed. Nur al-Din had gathered his armies and got them ready, and was awaiting the arrival of the news from Saladin that the latter had set out, so that he himself could [also] depart. When the news of that came to him, he set out from Damascus, resolved to make for Kerak. He arrived there and waited for the arrival of Saladin [who had forced the surrender of Shawbak and then withdrawn]. A letter from Saladin came to him, in which the former excused himself from coming to Nur al-Din because there were disturbances in the country [of Egypt], and he was afraid for the country because he was far from it. Saladin returned to Egypt, but Nur al-Din did not accept his excuse.
The reason for Saladin’s reticence was that his companions and leading officials had made him afraid of joining with Nur al-Din, and when Saladin did not follow the order of Nur al-Din the latter found that troubling and resolved to go down to Egypt and expel Saladin from it.
Ibn al-Athir goes on to describe how on this occasion Nur al-Din was mollified when Saladin, acting on the advice of his father Najm al-Din Ayyub, who was one of Nur al-Din’s most trusted lieutenants, wrote to Nur al-Din re-affirming his loyalty to him. Later, Ibn al-Athir gives the following assessment of the reasons behind Saladin’s reserve.
The thing that prevented Saladin from mounting an expedition [to fight the Franks] was fear of Nur al-Din, for he was convinced that when Nur al-Din had swept the Franks out of his way he would take the country [of Egypt] from him, so he was protecting himself with them and not undertaking their extermination. Nur al-Din was only interested in being serious about conducting an expedition against them with his [utmost] effort and capability, and when he saw Saladin failing to fulfil his obligations in the expedition and learned his [true] objective, he prepared to set out against him, but the order of God that cannot be resisted came to him [and he died].
Source: Ibn al-Athir. (1963) Al-Ta’rikh al-Bahir fi’l-Dawla al-Atabakiyya. Ed. ‘Abd al-Qadir A. Tulaymat. Cairo: Dar al-Kutub al-Haditha, pp. 158 and 161.