The time of the Soldier Emperors (235-284/85) is considered as a time of crisis. The ongoing, forcibly brought about change on the Imperial throne did not allow the Roman Empire to get some peace.
Moreover, it had to defend itself at various fronts against new, powerful enemies. Above all the Persian Sassanids in the east, then the Goths and the Sarmatians at the Danube. At the Rhine frontier, the Alamanni and Franks were fierce enemies. More and more often Rome withdrew troops from the Rhine frontier.
Around 250, Roman sources mention raids of the “Francii” into Gaul. Again and again, Franks and Alamanni raided Germania Superior, plundered the country and burnt down many forts along the Limes. The Franks were a young, large unit of Germanics. Various Germanic tribes on the right bank of the Rhine had melted, the Usipetes, Tencteri, Sugambri, Bructeri and others. Also the Alamanni were not a tribe in the sense of an ethnical unit either.
Raids into Roman territory
These raids were devastating for the Roman-Germanic provinces on the Rhine and Danube. As the remaining Roman troops often came too late, the Frankish and Alemannic warlords found more followers each time. Back then, most Roman cities did not have a city wall, except for the Colonia Claudia Ara Agrippinensium, today’s Cologne. The people in the villages, however, had no protection. Therefore, many left and moved into the interior of the Roman Empire. After decades of devastating raids, entire villages were abandoned, and large regions were deserted.
When the Sassanids attacked the Roman provinces in the east, Rome deployed troops from the Rhine to the East. Thereupon, the Franks invaded the province Germania Inferior in 256-258. They destroyed several garrisons along the Rhine and conquered Trier. Only Cologne with its city wall overcame the assault. Emperor Gallienus (253-268) re-conquered Trier and defended Gallia Belgica and Germania Superior. His governor in Germania Inferior, Postumus, marched against the Franks and could achieve a decisive victory. But in 260, Gallienus had to suddenly break off the campaign. His co-emperor in the east, his father Valerian, had been defeated by the Sassanid Persians and had been taken prisoner. As soon as the news reached Rome, a civil war broke out. Gallienus withdrew troops from the Rhine.
Again, the Franks and Alamanni crossed the Rhine. They destroyed all Roman forts, watchtowers and assaulted civil settlements along the Limes from the Rhine to the Danube. The raids were heavier than ever before.
The three Gallic provinces, Germania Inferior and Germania Superior, occasionally also Spain and Britannia, seceded from Rome and grouped together into a Gallic Empire (260-274, “Imperium Galliarum”). Postumus was proclaimed emperor by his troops; he conquered Colonia Claudia Ara Agrippinensium (Cologne) and made it the capital of the new empire. Altogether, it was a good time for Gaul and the Rhineland, and another period of prosperity for CCAA. High-quality gold coins with Postumus’ portrait have remained.
Finally, Emperor Aurelian (270-275) in Rome wanted to do away with the Gallic Empire and marched over the Alps with his troops. In 274, at Châlons-sur-Marne, Aurelian defeated the Gallic emperor Tetricus and restored Rome’s authority.
Yet, many soldiers had perished in the bloody battle, and no one remained to defend the Rhine border against the Germanic intruders.
Warlords overrun the country
Still in the same year, Frankish and Alamanni warriors crossed the Rhine again and penetrated deep into Roman territory. They conquered and plundered Cologne and Trier and devastated the country. Paris went up in flames. The romanized population left Agri Decumates, the land between the Rhine and the Danube. The garrison in Bonn still stood, but the remaining civilians gave up the villages and settlements outside and took cover in the garrisons.
For three years, anarchy prevailed, then emperor Probus (276-282) could defeat the Alamanni and the Franks. Now he took the border of the Roman Empire back to the rivers Rhine and Danube. In other words, Rome gave up the Limes and the Agri Decumates. At the same time, he made the Franks and Alamanni Roman allies (foederati). They could settle in Roman territory or directly at the border, and Franks settled the deserted areas on the left bank of the Rhine. In turn, they had to stand loyally to the Empire and defend it if necessary.
For decades, there was peace on the Rhine frontier.