As we have seen in the preceding article, Julius Civilis and the Batavians had reached everything they wanted: an independence that would be recognized by Vespasian (provided that he won the civil war against the emperor Vitellius), and revenge for the oppressive recruitment by the Romans and the death of Civilis' brother.
The only thing they should never do, was attack the base of the two Roman legions at Vetera - no emperor could leave an attack on this symbol of Roman power unpunished. If only one spear would be thrown across the walls of the legionary base, it was inevitable that a large army would come to the north and make up for the humiliation. Of course, the civil war had to be over, but whoever would be its victor, he was obliged to punish the attackers. Everybody knew that almost three years before, the Jews had attacked the Twelfth legion Fulminata, and that the Romans had retaliated ferociously. Julius Civilis, who had fought in the Roman auxiliaries and was a Roman citizen, certainly must have known.

And yet, at the end of September 69, the Batavians launched an attack on Vetera, or, to use its ancient name, Vetera. The moment was well-chosen: one month earlier, Vespasian's legions had invaded Italy. If there was to be a Roman retaliation, it would be postponed for some time. So, Julius Civilis died his hair red, and swore that he would let it grow until he had destroyed the two legions. What made him sign his own death sentence, we simply do not know.
Whatever the reasons, the Batavians were well-prepared, because they had received the best of all possible reinforcements: the eight auxiliary units that had fought for Vitellius in Italy in the Spring, were sent back to defend the Rhine, and had been recalled for the struggle against Vespasian (above). In the preceding year, they had fought against the levies of Caius Julius Vindex (above), and still earlier, they had been stationed in the war zone in Britain. These men knew how to fight, and had more battle experience than most legionaries. Civilis' messenger had reached them while they were already marching to the Alps, and easily convinced them that they had to side with the independent Batavians.
Let us, before we discuss the Batavian attack on Vetera, see what had happened to the eight auxiliary units. The supreme commander of the Roman forces in Germania Superior and Germania Inferior, Marcus Hordeonius Flaccus, had allowed them to pass Mogontiacum or Mainz.

He called his tribunes and centurions together and consulted them on the desirability of bringing the insubordinate troops to heel by force. But he was not by nature a man of action, and his staff were worried by the ambiguous attitude of the auxiliaries and the dilution of the legions by hasty conscription. So he decided against risking his troops outside the camp. Afterwards, he changed his mind, and as his advisers themselves went back on the views they had expressed, he gave the impression that he intended pursuit, and wrote Herennius Gallus, stationed at Bonn in command of the First legion, telling him to bar the passage of the Batavians and promising to follow closely in their heels with his army. The rebels could in fact have been crushed if Hordeonius Flaccus and Herennius Gallus had moved from opposite directions and caught them between two fires. But Flaccus abandoned his plan, and in a fresh dispatch to Gallus warned him not to molest the departing units.
It is unclear what really happened. Tacitus obviously blames Flaccus for not destroying the eight units, but things were more complicated than he indicates. We must remember that Germania Inferior, which was threatened by the Batavians, was not an important province; Germania Superior and Gallia Belgica, however, were. Probably, Flaccus wanted to leave the problem in the periphery, and allowed the Batavians to return home. Then, the war would remain somewhere in the north, where it did not threaten vital Roman interests. This attempt to localize the war where it did not hurt could have been a successful strategy, but, as we will see below, Flaccus was murdered, after which everything went wrong.
A second point is that both Roman armies in Mainz and Bonna (modern Bonn) were smaller than the eight auxiliary units. Only when Flaccus and Gallus were able to attack simultaneously, they were in the majority and could be victorious. Flaccus could not afford that both armies were defeated. Finally, there was a more important war going on in Italy, and he could not move too far to the north. So he decided upon this strategy: keep the vital base of Mainz at all costs, try to keep Vetera, and wait until the civil war is over.