Arsenals and Anarchy: Defense Modernization, Unstable Multipolarity, and the Risk of U.S.-China War

The U.S.-China rivalry is held in place by four bars, not three. John Mearsheimer’s (2026) “Iron Cage of International Anarchy” identifies anarchy, latent power, and the regional distribution of military power as the structure that makes great power competition relentless and East Asia unusually war prone. An additional bar, the efficiency with which each state converts wealth into usable military power, now matters more for the next two decades than the slow drift of wealth and population.

This paper builds a Defense Modernization Index (DMI), applies it to the United States and China from 1991 to 2026, and projects four scenarios to 2076 with a model of the regional balance. China closed the modernization-quality gap from roughly one-third of the U.S. level to near parity. The two powers are hollow in different places: Washington in its industrial base, Beijing in its high command. In every scenario the regional balance favors China into the late 2030s, and the sharpest danger window opens not when Chinese power peaks, but when it begins to decline. The main recommendation is to manage transitions, not levels: build deterrence by denial without an arms race, offer conditional assurance, bind crisis channels, and settle disputes without new faits accomplis, all before the danger window opens.

Keywords: defense modernization; conversion; unstable multipolarity; danger window; U.S.-China rivalry