The Mongols’ invasion of the territories of the Islamic world, especially Baghdad, the seat of the Islamic Caliphate, had great consequences, including the fall of Baghdad and the Bani Abbas Caliphate, the killing of the Caliph, and extensive destruction in the geography of their invasion. About half a century after the invasion of the Mongols, Ibn Taymiyyah (1263-1328), regardless of historical documents, accused Nasir al-Din al-Tusi, a prominent thinker, of collusion and cooperation with Hulagu Khan. After Ibn (...) Taymiyyah Harrani, these accusations were followed and disseminated in religious, historical, rijal (the study of hadith narrators and their personality), and even moral books by his students, such as Ibn Qayyim al Jawziyya (1292-1350). This article is an attempt to deny these accusations. Relying on the earliest historical documents, it tries to describe the historical event as it happened. In this regard, the article first provides the text of Nasir al-Din al-Tusi’s short treatise “On the Baghdad Incident”, included in Aladdin Atamalik Juveyni’s “Tarikh-i Jahangusha”, and then presents its descriptive interpretation separately. Although this treatise is short, it contains all the important points, from Hulagu Khan’s decision to go to war to the conquest of Baghdad and the analysis of the internal factors of the fall of the caliphate. This treatise, above all, is a very solid argument. For its author first of all was the witness of the Baghdad events. Secondly, in this writing, Nasir al-Din al-Tusi examines the Mongols’ attack on Baghdad but does not mention his own activity or role in the attack on Baghdad. Thirdly, if he had an activity and a role in this incident, or if he had an intention to take revenge on his religious opponents considering the situation, he should at least have hinted and expressed his approval, which is not the case at all. (shrink)