This article explores the ideological motivations for ISIS's prejudice against the Kurds. From the group's inception, ISIS has rejected any kind of understanding of Islam but its own. However, its animosity toward Kurds has its own purpose and foundations, separate from its religious dogmas. The aim of this article is to study the reasons behind its fundamental aversion toward the Kurds. An analysis of the literature of the Islamic State in Iraq and Syria (ISIS), and particularly their interpretation of the Qurʾan and of Islamic prophetic traditions and history, demonstrates such a clear prejudice toward the Kurds, that no honest researcher can deny the vehemence of the group's hatred of the Kurds and their nationalist ideology. This animus explains, at least in part, why ISIS promulgated such a virulent anti-Kurdish ideology and recruited fighters to invade Kurdish lands. This study examines the ideological motives behind the group's behavior toward the Kurds. The article's hypothesis is that the Arab extremists that filled the ranks of ISIS, whether they came from secular nationalist, jihadist, or even moderate Islamist backgrounds, shared an antipathy against the Kurdish people that was deeply rooted in pro-Arab nationalistic sentiment. Below, the article looks at the deadly combination of Baʿthist ideology, which had previously led to genocidal campaigns against Kurds under Saddam Hussein, with the religious extremism of ISIS. A qualitative research method of discourse analysis is used to shed light on the ideological roots of ISIS's antagonism against Kurds. Kurdish-studies researchers will benefit from this study, as well as security, counter-terrorism, and Middle Eastern researchers and students.