High Roman Gaul came to an end in an empirewide crisis characterized by foreign invasions and a rapid succession of rulers, as increased pressure on the empire’s frontiers exacerbated its internal economic and political weaknesses. Priority was given to holding the Danube and the East; despite sporadic visits by emperors, the West was neglected. In 260 and 276 Gaul suffered depredation by two recent confederations of Germanic peoples, the Alemanni and the Franks (facing Upper and Lower Germany, respectively). The ensuing civil war left Gaul, Britain, and (for a while) Spain governed by a line of “Gallic” emperors (beginning with Postumus [reigned 260–268]). These lands were reconquered by the Roman emperor Aurelian in 274, though there was further revolt about 279–80. Although unity was reestablished and order of a sort restored by Aurelian (reigned 270–275), Probus (276–282), and Carinus (283–285), the country was much altered. For example, about 260 the Agri Decumates were abandoned, and, from about the reign of Probus, there began an extensive program of city fortification, though on very restricted circuits that cut through, and even used as building material, the proud structures of the previous age. The countryside was prey to marauding peasants. There was, however, no move to exploit the crisis to gain independence: the “Gallic Empire,” though closely involving leading Gallic civilians, depended on the loyalty of the Rhine army; it thus championed Gallo-Roman, not Gallic, interests (essentially, the maintenance of a strong Rhine frontier).

After Diocletian and his successors radically reformed the empire in the late 3rd and early 4th centuries, Gaul enjoyed a new stability and even an enhanced role in imperial life. The reason for this was the empire’s renewed commitment to defend Italy from the Rhine. To ensure the loyalty of the Rhine garrison and the civil population that depended on it for protection, imperial representation in the frontier region became permanent. An official of the highest rank, a praetorian prefect, was based there, and a series of emperors and usurpers (in particular, Constantine I [reigned 306–337], Julian [355–363], Valentinian I [364–375], Gratian [375–383], and Magnus Maximus [383–388]) resided there for at least part of their reigns. Their seat of government was usually Augusta Treverorum (now Trier, Germany), the former civitas-capital of the Treveri and capital of Belgica, now “the Rome of the West.” (An interesting exception to the rule was Julian, who, with Trier rendered inhospitable by war, wintered in Paris, giving that city its first taste of future greatness.) Throughout the 4th century and especially in its latter half, the ever-present German menace as well as internecine strife occasionally caused the Rhine frontier to be broken, but it was always vigorously restored.

Some recovery of economic prosperity occurred, though it was fragile and uneven. The levying of taxes in kind rather than in cash may have weakened commerce, and the settlement of captive barbarians on the land indicates a rural labour shortage. Trier was endowed with magnificent buildings, but most Gallic cities failed to recover their Classical grandeur. The well-to-do, who were for the most part probably not descended from the aristocracy of high Roman Gaul (destroyed in the 3rd-century crisis), had loftier ambitions than their predecessors. Looking beyond the civitates, they eagerly sought posts in the imperial administration, now conveniently close at hand, basing their claim to advancement on their learning. (Gallo-Roman education, drawing vitality from the Gallo-Celtic love of eloquence, had long been renowned, but it blossomed fully in the 4th century in famous universities such as the one at Burdigala [Bordeaux].) As the century progressed, some educated Gauls grew extremely powerful; the best-known, Ausonius (c. 310–c. 393), a poet and professor at Burdigala, was appointed tutor of the future emperor Gratian and became his counselor. These worldly aristocrats, when not at court, favoured the country life; the latter 4th century saw the rise of the palatial villa, especially in the southeast. Other Gauls looked to serve an even higher power; Christianity, thought to have been introduced in the region about 250 by St. Denis of Paris, took root deeply in the land in the century following. An episcopal hierarchy (based on the Roman provinces and civitates) was developed, and monasticism was introduced by Martin of Tours (c. 316–397).