After the World War I, the Assyrians, who had previously lived in the mountainous Hakkari region of what is now eastern Turkey, relocated at British insistence to the Kurdish region of the new Kingdom of Iraq. During the war, the Assyrians had fought for the Entente against the Ottomans and suffered a genocidal reprisal as a result. Consequently, their position in Turkey was untenable once the war ended. Various conditions caused acrimony to develop among the Assyrians, the local Kurds, and the Iraqi state. That animosity eventually culminated in the Simele Massacre by the Iraqi army against Assyrian villagers in August 1933. The state used that event to build Iraqi national unity. This paper will show how actors of the nascent Iraqi state used ethnic cleansing as a tool for state-building. Iraqi state actors cast the Assyrians as agents of the British mandate and foreign enemies against which the Iraqis defined themselves. Rather than building an accusatorial case to prove ethnic cleansing, this paper seeks to understand the thinking and contingencies that caused state actors to become perpetrators of mass violence, and specifically how the state and the larger society used such violence to establish or reinforce their own burgeoning nationalist narrative. Sources include British and Iraqi perpetrators and observers. Iraqi government sources, including the king himself, viewed ethnic diversity as an impediment to national unity. The local press painted the perpetrators as defenders of the state against foreigners. British army officer R.L. Stafford described the general political climate leading to the massacre, as well as details of the aftermath, which included a triumphal return of the army to Mosul. Iraqi historian Khaldun S. Husry witnessed that triumph as a child and later relied on Stafford’s account to argue that Bakir Sidqi, head of the Iraqi army, had planned the massacre. While scholarship exists that treats the Simele Massacre from the perspective of the victims, there is a dearth of scholarly material examining it as a state-building exercise, possibly due to a reticence to examine ethnic cleansing from the perpetrators’ point of view. A few scholars, however, have touched on the subject, including Hursi and some who may be participating in this conference, whose work I consider as secondary sources, but there does not appear to be any scholarship focused on this massacre as an Iraqi state-building exercise.