A mixed group of barbarians that includes Vandals, Alans and Suebi crosses the Rhine into Gaul, thereby transgressing one of the Late Empire's most secure limines or boundaries, a climactic moment in the decline of the Roman Empire that initiates a wave of destruction of Roman cities and the collapse of Roman civic order in northern Gaul, and that occasions the rise of three usurpers in succession in the province of Britannia; hence the crossing of the Rhine is a marker date in the Migrations Period.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.

A frozen Rhine, making the crossing easier, is not attested by any contemporary, but was a plausible surmise of Edward Gibbon.

On the east bank, the mixed band of Vandals and Alans fight a raiding party of Franks.

The Vandal king Godigisel is killed, but the Alans come to the rescue of the Vandals, and once on the Roman side, they meet with no organized resistance.

Stilicho had depleted the garrisons in 402 to face Alaric in Italy.A roughly contemporary letter of Jerome, written from Bethlehem, gives a long list of the barbarian tribes involved, some of them, like Quadi and Sarmatians, drawn from history or literary tradition.

He lists the cities now known as Mainz, Worms, Rheims, Amiens, Arras, Thérouanne, Tournai, Speyer and Strasbourg as having been pillaged.

Jerome’s mention of Mainz first in a list of the cities devastated by the incursion is the sole support for the common assumption that the crossing of the unbridged Rhine was effected at Mainz.