Flaccus and Vocula did not have to be clairvoyants to know that the Batavian leader would try to catch them off-guard. And they could also surmise that he would do this on a moonless night, like the night of December 1/2, 69. Tacitus, however, wants us to believe that the attack of the eight Batavian auxiliary units came unexpectedly.
Vocula was unable to address his men or deploy them in line of battle. All he could do when the alarm sounded was to urge them to form a central core of legionaries, around which the auxiliaries were clustered in a ragged array. The cavalry charged, but were brought up short by the disciplined ranks of the enemy and forced back upon their fellows. What followed was a massacre, not a battle. The Nervian auxiliary units, too, were induced by panic or treachery to expose the Roman flanks. Thus the attack penetrated to the legions. They lost their standards, retreated within the rampart, and were already suffering heavy losses there, when fresh help suddenly altered the luck of the battle.
Some Basque auxiliary units [...] had been summoned to the Rhineland. As they neared the camp, they heard the shouts of men fighting. While the enemy's attention was elsewhere, they charged them from the rear and caused a widespread panic out of proportion to their numbers. It was thought that the main army had arrived, either from Neuss or from Mainz. This misconception gave the Romans new heart: confident in the strength of others, they regained their own. The pick of the Batavian fighters -at least so far as the infantry was concerned- lay dead upon the field; the cavalry got away with the standards and prisoners taken in the first phase of the engagement. In this day's work casualties in slain were heavier on our side, but consisted of poorer fighters, whereas the Batavians lost their very best.
Again, Tacitus' description is misleading to the extreme. Of course the Basque units did not arrive by accident, as Tacitus seems to imply. It must have been sent by Flaccus. Likewise, the suggestion that the Nervians 'betrayed' the Romans, is a marvelous example of Tacitean innuendo.
The battle of Krefeld was an important Roman victory, although the losses were enormous. This is corroborated by a macabre archaeological discovery: many dead people and horses did not receive a decent cremation, but were hurriedly buried in a large mass grave.
The consequences of the battle were enormous. The eight Batavian auxiliary units now disappear from Tacitus' narrative, although he uses the expression cohortes once in a non-technical sense. Civilis had shown his true intentions and lost his best men, and nothing withheld the Romans from marching on Vetera and lifting the siege.
The camp's walls were strengthened, the ditches deepened, supplies brought in, the wounded taken away. But there was no opportunity to invade the country of the Batavians and retaliate, because bad news arrived from the south: the Usipetes and Chattians, German tribes from the east bank of the Rhine, had crossed the river, were plundering the country and tried to besiege Mainz. It did not seem very serious, but it was prudent not to take any risks. After all, Mainz was more important than the camp in the north.
Therefore, the expeditionary force, strengthened with 1,000 soldiers from Vetera, returned. Immediately, Civilis renewed the siege of an undergarrisoned but better equipped Vetera. When his cavalry attacked the retrearing army near Neuss, however, they were soundly defeated.
The legionaries had shown their worth at Krefeld and Vetera, and when they reached Neuss, there was a pleasant surprise: Flaccus distributed money to celebrate the accession of Vespasian. As loyal adherents of Vitellius, this was more than they had expected. These were the days of the Roman carnival, the Saturnalia, and the legionaries celebrated it with pleasure. It must have come as some sort of release after the tensions of the preceding weeks. However, the merrymaking was disturbed.
In a wild riot of pleasure, feasting and seditious gatherings at night, their old enmity for Hordeonius Flaccus revived, and as none of the officers dared to resist a movement which darkness had robbed of its last vestige of restraint, the troops dragged him out of bed and murdered him.
The same would have happened to Vocula if he had not been able to make his escape from the camp, dressed as a slave. The assault on the two commanders at the moment when Fortune was smiling at the Romans, is one of the unexplained events during the Batavian revolt. We can only speculate about the reason. As we have already seen, when the Roman expeditionary force had returned to the south, it had taken men from Vetera with them. Tacitus mentions that those that were left behind felt themselves betrayed, and understandably so: they were to keep the defeated Batavians occupied while the main force was occupied somewhere else. Is it possible that the murder was not an act of drunken hysteria, but 'fragging', i.e., the killing of a commander who was careless with his soldiers?