Differentiated Alliance Management: The Generative Logic of Institutional Divergence in “US-Japan+” Trilateral Security Cooperation

Since 2022, three trilateral mechanisms anchored on the US-Japan alliance have advanced in parallel: US-Japan-Australia, US-Japan-Republic of Korea (ROK), and US-Japan-Philippines. Yet they have diverged sharply across four dimensions: organization, process, capability, and network. The three share the same US-Japan axis and converge on the China challenge, so one might expect their institutional forms to converge as well. They have not. This divergence is the puzzle the article addresses. To explain it, the article proposes “differentiated alliance management.” Washington and Tokyo, the argument runs, do not apply a uniform template. They adjust commitment form, organization, capability integration, and network connectivity to fit each third party’s domestic-political, capability, and diplomatic-risk constraints. Process tracing across the three cases shows that the same axis, filtered through different constraints, produces three stable but distinct forms: a non-alliance yet steadily deepening high-level integration with Australia, a legally non-binding political commitment with the ROK, and a selectively activatable, capability-building arrangement with the Philippines. It argues that Washington and Tokyo have not applied a uniform template when building trilateral cooperation with third parties; rather, they have calibrated the institutional configuration of each mechanism according to the domestic political, capability-structural, and diplomatic-risk constraints that each partner faces.