e Persian Sassanid dynasty in 226 threatened to go to war much more often than it engaged in hostilities. The Roman legions constituted a magnificent fighting machine, noteworthy for their courage, discipline and cohesion as an aggressive fighting force. As a result, Starr said "eventually every revolt in the first two centuries was suppressed. Open challenge to the imperial system was not permitted" (121).
Under Augustus, the legions had conquered Asia Minor, Greece, the Aegean islands, Thrace which commanded the approaches to the Bosphorus, and the Dalmatian coast. Later Caesars, notably Vespasian (69-79) and Trajan (98-117), expanded further in the lower Danube region. Most of the boundaries of pre-1990 western Yugoslavia fell under Roman control. Border control was maintained from fortifications built along the lower Danube in Bavaria (Raetia), Austria (Nordicum), in Upper Pannonia in the Vienna-Budapest region and in Lower Pannonia between Budapest (Aquincum) and south of Belgrade (Carnuntum). In the two Dacian wars, Trajan crossed the Carpathians and defeated an ancient steppe people, the Dacians, in Transylvania which attracted the
Romans because of its gold mines. However, Mommsen said that "Dacia remained an advanced position on the left bank of the Danube" which remained "the limit of Roman civilization" (240).
Marcomannian Wars. Various Germanic tribes, including the Marcomanni and Quadi in Bohemia and Moravia, disrupted the Roman lower Danubian frontier in upper Pannonia and in 170 invaded northern Italy. They were finally pushed back by the Emperor Marcus Aurelius (161-180) who conquered most of Hungary in the process. Barbarian raids by various nomadic steppe peoples troubled the frontier all along the length of the Danube. The Goths were not directly involved, but their migrations south from Poland were believed to have squeezed the Vandals and Langobards out of the upper Vistula who in turn placed pressure o...