The crossing of the Rhine River by a mixed group of barbarians which included Vandals, Alans and Suebi is traditionally considered to have occurred on the last day of the year 406 (December 31, 406). The crossing transgressed one of the Late Roman Empire's most secure limites or boundaries and so it was a climactic moment in the decline of the Empire. It initiated a wave of destruction of Roman cities and the collapse of Roman civic order in northern Gaul. That, in turn, occasioned the rise of three usurpers in succession in the province of Britannia. Therefore, the crossing of the Rhine is a marker date in the Migration Period during which various Germanic tribes moved westward and southward from southern Scandinavia and northern Germania. Ancient sources Several written accounts document the crossing, supplemented by the time line of Prosper of Aquitaine, which gives a firm date of 31 December 406 in his year-by-year chronicle: "In the sixth consulship of Arcadius and Probus, Vandals and Alans came into the Gauls, having crossed the Rhine, on the day before the kalends of January." A letter by Jerome, written from Bethlehem and dated to the year 409, gives a long list of the barbarian tribes involved (Quadi, Vandals, Sarmatians, Alans, Gepids, Herules, Saxons, Burgundians, Alemanni and the armies of the Pannonians). Some of them, like Quadi and Sarmatians, are drawn from history or literary tradition. Jerome lists the cities now known as Mainz, Worms, Rheims, Amiens, Arras, Thérouanne, Tournai, Speyer and Strasbourg as having been pillaged. In his History of the Franks, the 6th-century historian Gregory of Tours embedded some short passages of a lost account by the 5th-century historian Renatus Profuturus Frigeridus of a war between the Vandals, Alans and Franks that took place in the neighbourhood of the Rhine around the time of the supposed crossing of the Rhine. This text, scholarly called the "Frigeridus fragment", may provide some clues about the circumstances preceding the crossing. Olympiodorus of Thebes, a generally reliable contemporary historian, wrote an account of the crossing, of which only fragments have survived in quotations by Sozomen, Zosimus and Photius. Orosius mentioned the crossing in passing. Interpretation Motives The initial gathering of barbarians on the east bank of the Rhine has been interpreted as a banding of refugees from the Huns or the remnants of Radagaisus' defeated Goths, without direct evidence. Scholars such as Walter Goffart