How where the Germanic groups able to cross into Gaul in ad 406?

The Vandal, Sueves and Alans crossed the river Rhine in the winter, when it was frozen. This made the crossing easier. This river was the frontier of Gaul. In addition to this, its frontier garrisons were undermanned because many Roman troops had been redeployed to Italy the previous year to fend off a major invasion of Italy by Redegasius, an Ostrogoth king.