The Lower Empire
« Atlas historique Larousse » sous la direction de Georges Duby, édition revue et corrigée, 1987
In the 3rd and 4th centuries, the Empire faced a triple threat: to the north, the Germans, numerous, unstable, and warlike, pressed on its borders; to the east, the Sassanid Persians, victorious over the Arsacid Parthians, constituted the only organized state opposing Rome; in the south, the Saharan nomads posed the least dangerous threat. The crisis reached its peak from 256 to 269, when these enemies coordinated their attacks. However, from Claudius II (268–270) to Diocletian (284–305), the Illyrian Emperors managed to stabilize the situation, despite significant challenges (such as the Alamanni and Franks invading Gaul in 275). At times, they negotiated deals, notably with the Sassanids. In 332, the Goths from the lower Danube, defeated, agreed to an unequal alliance treaty with Constantine. In exchange for subsidies and the right to trade with Rome, they pledged to provide troops to the Emperor upon request. They remained loyal to the Constantinian dynasty thereafter, supporting Constantius II against Magnentius in 351-353 and backing the attempted usurpation of Procopius against Valens in 365.
Constantine organized the first universal council in 325 at Nicaea, bringing together bishops to achieve theological unity within Christianity. Some external bishops (Persians and Goths) attended, and Constantine, as Roman Emperor, presented himself as the natural protector of Christians worldwide.
Constantine’s establishment of Constantinople as the second capital shifted the empire’s center of gravity (331). Constantine’s three sons succeeded him, and after several episodes of civil wars, Constantius II (337-361) emerged victorious from the conflicts. He imbued power with a hieratic, sacred, and Christian character.
However, stabilization primarily came from the reorganization of the army (stationing fixed units at the borders and maintaining a mobile reserve at the rear), leading to victories over Persians, Goths, and Franks, ensuring half a century of tranquility. This peace was reinforced thanks to the resolute character of Valentinian I (364–375).