This paper examines a major paradigm shift in Catholic Social Teaching on war and peace, arguing that the contemporary move from just war to just peace is not merely a pastoral adjustment but a deeper transformation in Catholic normative reasoning. Situating this shift within the tradition's broader engagement with social inequality, the paper shows how inherited moral frameworks are reconfigured in response to historical change, empirical experiences of violence, and power asymmetries. Using a comparative hermeneutical approach, the paper places the twentieth-century Catholic re-reading of the Augustinian bellum iustum tradition—from Benedict XV through Vatican II to recent papal teaching—into dialogue with rabbinic interpretive dynamics that prioritize peace through inversion and recontextualization. The turn toward just peace is interpreted as the retrieval of a previously marginalized theological horizon: an Augustinian theology of peace long implicit beneath the formal structures of just war theory. By highlighting the methodological transition from a classicist to a historically conscious moral framework, the paper analyzes how Catholic Social Teaching negotiates the tension between universal normative claims and concrete social conditions marked by inequality, structural violence, and technological escalation. The emergence of just peace discourse illustrates how Catholic moral reasoning challenges entrenched hierarchies of legitimacy, especially those normalizing coercion, while also introducing new ambiguities when ethical responsibility shifts from fixed criteria to historically situated discernment. Overall, the paper presents Catholic Social Teaching as a living and contested discourse, capable of critiquing inequality and violence yet continually shaped by the unresolved tension between authoritative tradition, historical context, and the ethical demands of peace.