The origins and effects of anti-Iranian sentiment on and within Iraqi politics, citizenship laws, and nationalisms is a well-studied subject in modern Iraqi studies. So too the long history of border conflicts between Iraq and Iran. However, there is currently no study of the link between how that border functions as a state institution and broader anti-Iranian (and anti-Shi?a) sentiment. If we take this connection for granted, we risk, on the one hand, granting racist or sectarian narratives too much influence over a multitude of governmental actions, and on the other hand, we will fail to account for the highly contingent nature of boundary enforcement processes over time and from place to place. To address this gap in the historiography of nationalism, sectarianism, and border studies in Iraq, this paper focuses on the criminalization of Iranian nationals (or those of alleged Iranian background) through the process of boundary enforcement. In other words, this paper asks, What is the relationship between codified laws, enforcement policies, and pre-existing or concomitant popular or official Iraqi nationalisms? Using British and Iraqi archival documents (including those of the Iraqi Baath Party) and press records stretching from the period of colonial Mandate rule after World War I, to the Iran-Iraq War of the 1980s, I show that the day-to-day administration of the boundary during war-time and peace-time was used, when expedient, to effectively criminalize being Iranian in Iraq at key moments in Iraqi political history. I approach these sources, and the Iraq-Iran borderland in general, as the manifestation of a revealing discourse between bureaucratic, legalistic practices and fundamentally subjective national-territorial narratives of “us” and “them.” In the early 1920s, British and Iraqi government forces sought to discredit and defeat localized revolts contiguous to, and overlapping, the border between Iraq and Iran. Likewise, in the 1980s, the ruling Iraqi Baath Party sought to maintain hegemonic control in northern Iraq amidst Iranian invasions and more local rebellions within the borderland. In both cases, the threat of the Iranian “other,” as a sympathetic and violent “fifth column” within Iraq, drew state attention to the Iraq-Iran border and its enforcement. Iranians were subsequently made “illegal” in Iraq, subjecting them to state violence, societal stigmatization, and further normalizing Iranian and Shi?a “otherness” in Iraq, and elsewhere in the Arab world.