In this paper, the mechanical behavior and performance of the onager, an ancient war machine, are analyzed. This study is based on torsional motors, as previously described. Because the onager has been investigated very little in modern times and the details given by ancient engineers are very scarce, this study starts from the available literature of both ancient and modern times. The mathematical models are developed through analytical mechanics along with integrations carried out numerically. Simulations performed provide interesting results which are in good agreement with the information recorded by ancient historians, and thus validating conjectures regarding the design of these machines. Finally, this investigation clarifies the role and effectiveness of these machines in ancient warfare, helping to understand many aspects of both ancient engineering design and historical events.