Augustine is considered the first theologian to conceive and develop a distinctly Christian ‘just war’ theory that legitimizes war. In his theory, Augustine ad- dresses the cause, intention, and specific conditions that make war justifiable. The problem, however, is that Augustine’s theory of just war has been employed to justify wars such as the medieval crusades, and it is now used to legitimize modern warfare such as the ‘war on terrorism.’ In this context, this paper explores whether the war on terrorism can actually be justified based on Augustine’s theory and whether it is effective as a response to terrorism. To this end, the study first deals with Augustine’s concept of ‘just war,’ then examining its historical interpretations and applications, and finally discussing the justifications for and effectiveness of the war on terrorism. Through this process, the paper argues that since jus in bello (justice in war), Augustine’s prerequisite for just war, is not carefully actualized in counterterrorism warfare and the continuous use of military force cannot resolve the root causes of terrorism, the war on terrorism cannot be claimed to be justified and nor is it effective. Therefore, it is suggested that non-violent actions such as international cooperation and humanitarian efforts that can mitigate or eliminate the causes of terrorism and thus have a more positive effect on the situation of terrorism should be reflected as key principles in counterterrorism policy.