When the plot came to nothing, Civilis resorted to force and enrolled the Cananefates, Frisians and Batavians in separate striking forces. On the Roman side, a front was formed at no great distance from the Rhine, and the naval vessels which had put in at this point were arrayed to face the enemy. Fighting had not lasted long before a Tungrian unit went over to Civilis, and the Roman troops, disarrayed by this unforeseen treachery, went down before the combined onslaught of allies and foes.
The Tungrians where a romanized tribe that lived in the east of what is now Belgium, where their name lives on in the town called Tongeren. To the Romans, their desertion during this battle (which must have taken place south of modern Arnhem) was alarming, because it suggested that auxiliary units that were recruited among otherwise loyal tribes, could be unreliable. However, they and the depleted legions were the only soldiers Flaccus could use. Even worse, volunteers from the northern provinces and the German tribes across the Rhine sided with Civilis.

This success earned the rebels immediate prestige, and provided a useful basis for future action. They had obtained the arms and ships they needed, and were acclaimed as liberators as the news spread like wild-fire thought the German and Gallic provinces. The former immediately sent an offer for help. As for an alliance with the provinces of Gaul, Civilis used cunning and bribery to achieve this, returning the captured commanders of the auxiliary units to their own communities and giving the men the choice between discharge and soldiering on. Those who stayed were offered service on honorable terms, those who went received spoil taken from the Romans.

The Romans were now expelled from the country along the rivers Maas, Waal, and Rhine. The cavalry base at the Kopse Hof is the only Roman camp that has no burning layer, which suggests that the Romans were able to keep it, and still controlled the Waal crossing near Nijmegen.
Up till now, the war had, on the Roman side, been waged by auxiliaries: lightly-armed troops that were recruited among the native population, and were no match for the Batavians, who were in the majority. Flaccus' reply to their defeat was to send in the legions, heavily-armed infantry men. The Fifth legion Alaudae and the Fifteenth legion Primigenia left their base at Vetera, together with three auxiliary units: Ubians from modern Cologne, Trevirans from modern Trier, and the Batavian squadron. Flaccus and the commander of the expeditionary force, a senator named Munius Lupercus, may have had his doubts about the latter, but they knew that it was commanded by a personal enemy of Julius Civilis, a man named Claudius Labeo. Late August, they invaded the Island of the Batavians. Somewhere north of Nijmegen, they encountered the Batavian army.
Near Civilis were massed the captured Roman standards: his men were to have their eyes fixed upon the newly-won trophies while their enemies were demoralized by the recollection of defeat. He also caused his mothers and sisters, accompanied by the wives and young children of all his men, to take up their station in the rear as a spur to victory or a reproach to the routed. Then the battle chant of the warriors and the shrill wailing of the women rang out over the host, evoking in response only a feeble cheer from the legions and auxiliary units. The Roman left front was soon exposed by the defection of the Batavian cavalry regiment, which immediately turned about to face us. But in this frightening situation the legionaries kept their arms and ranks intact. The Ubian and Treviran auxiliaries disgraced themselves by stampeding over the countryside in wild flight. Against them the Batavians directed the brunt of their attack, which gave the legions a breathing-space in which to get back to the camp called Vetera [i.e., Vetera].