Unsurprisingly, the arrival of the Vandal migrants led to conflict as the Franks and Alamanni attempted to close their borders. There were probably many small engagements as groups of new immigrants tried their luck, only to be repulsed. Most of these have gone unrecorded, but at some point there was a major battle between the Vandals and the Franks. Fragments of the contemporary writer Renatus Profuturus Frigeridus, preserved by Gregory of Tours, say that the Vandals were on the brink of a catastrophic defeat. Their king, Godegisel, was killed in the fighting but at the last minute the Vandals were saved by the timely intervention of a force of Alans under Respendial, who `turned the army of his people from the Rhine, since the Vandals were getting the worse of the war with the Franks, having lost their king, Godegisel, and about 20,000 of the army, and all the Vandals would have been exterminated if the army of the Alans had not come to their aid in time.’
This battle probably took place some time in the summer or autumn of 406, and it allowed the Vandals and their allies to move into Frankish territory on the middle Rhine. Although they had won a path to the Roman frontier, the new immigrants must have been in a fairly desperate state. Unable to grow or harvest crops and with no supply bases to call on, it would have been a monumental task to keep their people and livestock alive. The Vandals were a settled people with no nomadic history and no expertise in living off the land. If they managed to move up to the Rhine in the autumn of 406 they may have been able to take in some of the crops the Franks had planted, but this would at best only keep starvation at bay for a few months.
In the pre-industrial age armies rarely moved in a North European winter. Without the benefit of canned goods, mass production and mechanised transport that did not require forage, any movement of a large group of people in winter would inevitably lead to utter disaster. Yet the Asdings, Silings, Suevi and Alans crossed the Rhine in the depths of midwinter. What on earth persuaded them to do this when all sensible armies would have been in winter quarters awaiting the onset of the spring campaigning season?
The traditional view is that the winter was so cold that the Rhine froze over, giving the invaders the possibility to cross on a wide front. Although the Rhine remains open all year round in present times, it has frozen over in the past and it is not impossible that it froze in the winter of 406/7. Whether the ice would have been thick enough for tens of thousands of people with their wagons and baggage to cross is another matter. There are no contemporary accounts to support the idea of a crossing on ice, despite the fact that it has become a relatively accepted popular image.
The most evocative popular account of the crossing of the frozen Rhine is in Wallace Breem’s delightful novel Eagle in the Snow. Here we see the last remnants of the Roman frontier forces fighting a last stand, which is doomed as soon as the Rhine freezes. A story of civilization fighting off barbarism or established cultures digging in against impoverished migrants has a strong resonance today, just as it did in the eighteenth century when Edward Gibbon wrote his Decline and Fall of the Roman Empire. It was Gibbon who first gave us the story of the Rhine freezing over, possibly to explain his incomprehension at how the Vandals, Alans and Suevi were able to cross over into Gaul with such apparent ease. Many modern writers have followed Gibbon, although even he himself was not definitive about the river freezing: `On the last day of the year, in a season when the waters of the Rhine were most probably frozen [my italics], they entered, without opposition, the defenceless provinces of Gaul.’
In truth, the move of tens of thousands of people with all their belongings in the depths of midwinter must have been one of desperation. The Frankish lands the migrants had occupied on the east bank would not have sustained them for long. As winter began to set in, so would the prospect of starvation. On the west bank there were well-provisioned towns and a few probing raids would have given the Vandals and their allies some indication of the paucity of Roman defences, which had been stripped to support Stilicho’s campaigns in Italy.
The crossing of the Rhine probably took place at several points, with Mainz (Mogontiacum) as the centre of axis. It did not need the Rhine to freeze over to make such a crossing possible. The Roman Rhine bridges were still standing, and if the river was open then picked warriors in makeshift boats could have gone across first to secure the bridgeheads. If the river had frozen over then this could possibly have been done over the ice. If there was even partial freezing of the river, then the Roman Rhine fleet would not have been able to intervene.
Defending the Rhine
In order to understand how easily the Vandals, Alans and Suevi crossed the Rhine once they had defeated the Franks on the east bank, we need to appreciate the Roman system of defence. The frontiers, were defended by troops deployed in fortifications along the borders of the empire. Known as limitanei (soldiers defending the frontiers, or limes) or riparienses (soldiers defending the rivers), these men occupied strongpoints along the frontiers and patrolled the borders. Deployed in relatively small detachments, they were able to deter or intervene to deal with small-scale incursions but were neither expected nor able to deal with a major invasion. To think of them in modern terms, they were more like a border force or home guard than regular armed forces, even though many of the units could trace their heritage back to the legions of previous centuries. Backing them up, in the Western Empire, there were two main field armies, one based in Italy and the other in Gaul. Each of these field armies were, on paper, about 20-30000 strong and it was their job to intervene once the frontiers has been breached to defeat the invaders and restore order. The field army units were known as palatini – the most senior units who were originally part of a central army commanded in person by the Emperor – and comitatenses – units of the regional field armies. Units of limitanei drawn from the frontier to reinforce the field armies took on the title of pseudocomitatenses.
This was the theoretical principle of defence, but in reality the field armies in the fifth century were more occupied supporting various political interests than they were in defending the empire from external threats. Stilicho had the support of the Italian field army but not that of Gaul. When Constantine crossed over from Britain in early 407, the Gallic Army went over to him.
The Gallic field army was commanded by the Magister Equitum intra Gallias who in theory had twelve cavalry and forty-eight infantry units at his disposal, with unit strengths probably averaging out at roughly 500 men each. However, we have already seen how Stilicho struggled to field an army of thirty units to fight Radagasius and so we should not assume that the Magister Equitum’s entire force could be quickly and easily deployed. Furthermore, many of these units would have been severely weakened after their defeat in the civil wars and may well not have been anything like at full strength.
The defence of the middle Rhine, where the Vandals crossed, fell to the Dux Mogontiacensis (Duke of Mainz). According to the Notitia Dignitatum, he had eleven prefects under his command. The units commanded by these prefects and their home stations are recorded as:
Praefectus militum Pacensium, at Saletio (Seltz)
Praefectus militum Menapiorum, at Tabernae (Rheinzabern)
Praefectus militum Anderetianorum, at Vicus Julius (Germersheim)
Praefectus militum Vindicum, at Nemetes (Speyer)
Praefectus militum Martensium, at Alta Ripa (Altrip)
Praefectus militum Secundae Flaviae, at Vangiones (Worms)
Praefectus militum Armigerorum, at Mogontiacum (Mainz)
Praefectus militum Bingensium, at Vingo (Bingen)
Praefectus militum Balistariorum, at Bodobrica (Boppard)
Praefectus militum Defensorum, at Confluentes (Koblenz)
Praefectus militum Acincensium, at Antennacum (Andernach)
Some of these units are also listed under the Gallic field army, which may indicate that they had been pulled back from the Rhine. Alternatively, it could also mean that a few units of the field army such as the Menapii and Armigeri (senior legions of the Gallic field army and also listed under the Dux Mogontiacensis) had been sent to reinforce the frontier. However, as the trend had been to strip the frontiers to bolster the field armies it seems that the first possibility is the most likely.
These units probably only contained few hundred men each. While they could hold the walls of a fortified strongpoint and mount patrols, there could never have been any possibility that these dispersed garrisons could block a crossing of the frontier by many thousand warriors, even if the latter were weakened by hunger and bogged down with their families and chattels. At best all these men could hope for would be to hold out behind their fortifications while the barbarians moved past, to be dealt with by the field army at a later date.
There was also a Rhine fleet, the Classis Germanica, which patrolled the river and was an integral part of the Roman defensive system. We do not know how large it was, but in 359 Ammianus Marcellinus tells us that a squadron of forty ships was used against the Alamanni. Later inscriptions testify to ongoing clashes with Germanic tribes up until the Rhine crossing in 406. If the Rhine had been frozen or partially frozen, then it would have prevented any ships from intercepting the invaders. The Classis Germanica seems to have disappeared after the Vandal invasion as there is no mention of it in the Notitia Dignitatum.
So if the Dux Mogontiacensis and his border force were never expected to hold back a major invasion, what did the Gallic field army do?
Apparently very little.
In the fourth century the Gallic capital had been at Trier on the Rhine, but by the fifth century it had moved to Arles at the mouth of the Rhone in the south. The Rhine had been abandoned psychologically, if not yet in reality. From Arles, the focus of the authorities was far more towards Italy and the Mediterranean than to the northern frontiers.
The explanation for the apparent inaction by the Gallic Army rests in the convoluted Imperial politics of the time. On paper the Gallic Army should have had enough men to deal with the barbarian incursion across the Rhine. However, as the Vandals and their allies were crossing into Gaul from Germany, so too was Constantine from Britain. Unloved and run down by Stilicho, the Gallic Army threw in their lot with Constantine and their main worry was to hold their own against the Imperial authorities with the barbarian incursion a secondary concern.
Constantine probably crossed the channel in early 407, bringing with him the last remnants of the Roman Army in Britain. He forged alliances with the Franks and Alamanni and there is some evidence that he fought against the various bands of Vandals, Suevi and Alans to bottle them up in northern Gaul for a while.
Most of the military might of the Western Empire resided under Stilicho’s command in Italy. He was about to embark on a war with the Eastern Empire over control of Illyricum. So what did he do when he learned that the barbarians were overrunning Gaul and that Constantine was doing his best to contain them?
Naturally he did what any late Roman potentate would do. He sent an army to Gaul to destroy Constantine. A usurper was, after all, a far greater threat to his power than a mere barbarian invasion. Stilicho’s army, led by the Goth Sarus, was defeated, leaving Constantine in control of Britain and Gaul, with Spain also recognizing his authority. So it was, that rather than concentrating their forces to defeat a foreign invader the Romans fought amongst themselves and left the field open to the Vandals and their allies.