This study examines whether coalition command, control, communications, computer, and intelligence (C4I) systems interoperability is the next logical step for the US beyond joint interoperability. This study uses US experiences in the Korean War, DESERT SHIELD/STORM, Operation JOINT ENDEAVOR, plus past research to analyze and establish currents trends, patterns, and gaps in coalition interoperability. It is clear from previous operations and past research that the US, allies, and coalition partners have not mastered coalition C4I systems interoperability. In order to achieve coalition C4I systems interoperability, this study recommends a course of action (COA) to rely on common US C4I systems or adopt common technical standards to ensure coalition interoperability in the acquisition of C4I systems. Adopting this COA allows alliance and coalition partners to either purchase US C4I systems or design and procure C4I systems that will be interoperable based on common technical standards. Future operations will be multinational and coalition interoperability will be paramount to success. Training programs, combined exercises, military-to-military contacts, foreign military sales, coalition doctrine, modularity, the global grid, and Radiant Mercury (a tool for multilevel security) formulate conditions for success, thus making coalition interoperability the next logical step beyond joint interoperability when designing, procuring, and build US C4I systems.