The recent United Nation Secretary General’s report on sustaining peace speaks to an urgent crisis of complexity in global affairs, where a wide assortment of nonstate actors wields more political power than ever before. In this context, the international community’s traditional ways of forecasting, planning, policymaking, and assessing impact are becoming rapidly obsolete. In response, policymakers are calling for more holistic or systemic approaches to peace and development. Unfortunately, these proposed changes are merely ‘systems light’, essentially a metaphorical characterization of peace systems where their component parts are seen as interconnected and complicated. This form of systems thinking is insufficiently informed by more sophisticated methods from complexity science. This article will illustrate how two methods derived from complexity science, causal loop diagramming and mathematical modeling, can help us understand the properties and dynamics of intervention in complex peace systems. Causal loop diagrams help us to identify the peace factors and the connections between them. Mathematical modeling helps us determine the quantitative results of the interactions between all the peace factors. Using these methods together can lead to new insights for peacebuilding and for mitigating the unintended consequences of well intended policies.