a Roman province formed in 15 B.C. in the interfluve between the Rhine and the Danube in the Alps. Rhaetia was inhabited mainly by Rhaetian and Vindelician tribes. During the reign of Emperor Diocletian in the third century A.D., Rhaetia was divided into two provinces—Rhaetia Prima in the south and Rhaetia Secunda in the north. In the middle of the fifth century Rhaetia was conquered by the Alamanni, Suevi, and other Germanic tribes.