無 In the past decades, we are witnessing a rapid growth of defense cooperation agreements (DCAs) in contrast to the decreasing amount of newly formed alliances. I examine the formation of DCAs in the lens of contextual change and costly signals. In terms of contextual change, I construct the environment of the strategic interaction by specifying both structural and institutional context. Within these preceding contexts, I contend that DCAs are the product of two efforts by states. First, they are states’ adjustments toward unexpected contextual change, e.g.: global and regional structural change, institutional bargaining, or the combination of both. Second, they are states commitment to the existing international institutions as well as broader regional and international order. In both cases, states attempt to reduce the uncertainty in the international society. Accordingly, I conduct two case studies: the Quadrilateral Security Strategy in the Indo-Pacific and the European Common Defense Policy. By conducting both in-depth case studies and controlled comparisons, I conclude that given the rapid change of security challenges, states opt to strengthen existing regimes rather than engaging outright confrontations. Consolidation as such is achieved by signing DCAs to bolster the security and defense networks.