The Arch of Constantine is one of ancient Rome’s best-known monuments, famous for its use of spolia and its status as emblem of the religious and political changes that occurred under Constantine. Yet one of its most ground-breaking aspects is its explicit depiction of Constantine’s civil war against Maxentius. This study examines the historical context for this revolution in Roman art. By analyzing the Arch of Constantine’s topography, architectural form, sculptural decoration, and epigraphy in the context of similar imperial monuments, it argues that Constantine and the Senate intentionally subverted the longstanding norms of an imperial triumphal arch, transposing the implications of the Roman triumph onto Constantine’s civil war victory over Maxentius. It argues further that the two parties worked jointly to design the arch’s unprecedented representations of civil war because of a symbiotic desire to reject the Tetrarchy: Constantine sought to pursue sole power while the Senate hoped to reclaim Rome’s—and its—centrality in the empire.