On the last day of 406 a large coalition of barbarian groups crossed the Rhine. No surviving contemporary source tells us why. Two recent studies have accepted that large numbers of outsiders were involved in the action, but whereas I argued in 1995 that further westward movement on the part of the nomadic Huns was the likeliest trigger of the bigger crisis, Walter Goffart and Guy Halsall have suggested that divisions between the two halves of the western empire and/or an effective withdrawal of Roman power from the northwest encouraged the invasions. In the absence of direct evidence, all attempts to understand the crisis must take the form of argued hypothesis, and there is certainly room for more than one point of view. But all hypotheses must be judged on their capacity to account for the entirety of the evidence, and this study argues that because the first of these attacks (Radagaisus) affected Italy rather than the Rhine, then any withdrawal from the northwest (for which the evidence is weak) cannot have been the trigger. Likewise, although division between east and west certainly affected what happened afterward, it is an insufficient explanation for the original invasions, because the vast majority of the invaders who emerged from the middle Danubian region between 405 and 408 had not been living there in the fourth century. Any convincing explanation of the crisis also must account for what had brought large numbers of Alans, Goths, and Vandals west of the Carpathians in the first place, before they then invaded the Roman Empire. It is argued here that a desire on their part to escape the Hun-generated chaos and predation remains the best overall explanation.