Rebellion of the Seven States
The Han dynasty initially added a top level of "kingdoms" or "principalities" (王国, "wángguó"), each headed by a local king or a prince of the imperial family. From the establishment of the dynasty, however, the tendency was to slowly absorb this quasi-federal structure into the imperial bureaucracy. After the Rebellion of the Seven States, the system was standardized, replacing the kingdoms and principalities with thirteen provinces (州, "zhōu").