Southeast Asian states face numerous security challenges that require the assistance of external partners. China and India, two Indo-Pacific powerhouses, could offer potential solutions but their relations with Southeast Asian states vary considerably. At the same time, escalating tensions between China and India increase the risks of their engagement with Southeast Asian states leading to greater polarization in the region. By utilizing the "4-C Calculus", which comprises cost, complexity, credibility and capacity, this special issue seeks to understand how Southeast Asian states evaluate China and India as potential security cooperation partners, and whether cooperation with both—together or individually—can help address the region's security needs. The articles in this special issue employ the 4-C framework to analyse five key security concerns: defence modernization; health security; the post-coup crisis in Myanmar; humanitarian assistance and disaster relief; and maritime security. They contribute to the literature on security partnerships by providing fresh insights into our understanding of why and how smaller states partner with larger powers over shared security challenges, as well as by illustrating how certain policy considerations can influence the direction and quality of security partnerships.