Julius Civilis still commanded one of the Batavian auxiliary units in Roman service, and the commander of the Rhine army, Marcus Hordeonius Flaccus, did not know that Civilis conspired against Rome, although he suspected that something was going on. This offered Civilis an opportunity: he induced the Cananefates (the tribe that lived between the Batavians and the sea) to revolt, hoping that Flaccus would send him to suppress the rebellion. Tacitus tells how the war against the Romans started in August of 69.

Among the Cananefates was a foolish desperado called Brinno. He came from a very distinguished family. His father had taken part in many marauding exploits [...]. The mere fact that his son was the heir of a rebel family secured him votes. He was placed upon a shield in the tribal fashion and carried on the swaying shoulders of his bearers to symbolize his election as leader. Immediately calling upon the Frisians, a tribe beyond the Rhine, he swooped down on two Roman auxiliary units in their nearby quarters and simultaneously overran them from the North Sea. The garrison had not expected the attack, nor indeed would it have been strong enough to hold out if it had, so the posts were captured and sacked. Then the enemy fell upon the Roman supply-contractors and merchants who were scattered over the countryside with no thought of war. The marauders were also on the point of destroying the frontier forts, but these were set on fire by the commanders because they could not be defended.

Among the two camps that Brinno destroyed was that of the Third Gallic cavalry unit at Praetorium Agrippinae (modern Valkenburg near Leiden), where archaeologists have discovered the burning layer. Among the frontier forts that were destroyed by the Romans themselves, was Traiectum (modern Utrecht). A telling detail is the treasury of fifty gold pieces that was buried by a soldier who was never able to recover his money. Tacitus continues his story:
The headquarters of the various auxiliary units and such troops as they could muster rallied to the eastern part of the Island under a senior centurion named Aquilius. But this was an army on paper only, lacking real strength. It could hardly be otherwise, for Vitellius had withdrawn the bulk of the units' effectives.

By lucky coincidence, this Aquilius is known to us from an archaeological discovery: a silver disk or medal that was discovered in a cavalry base (the so-called 'Kopse Hof') east of the Oppidum Batavorum, the capital of the Batavians (modern Nijmegen). The man's full name was Caius Aquillius Proculus, and he belonged to the Eighth legion Augusta, which was not stationed in the German provinces.
This is a very important find, because it vindicates the Roman general Flaccus: if a senior centurion was present in Nijmegen, Flaccus had already sent reinforcements, which can only be explained if we assume that he expected trouble among the Batavians. Tacitus' story that Brinno's attack was a surprise, is misleading: the Romans were indeed caught off-guard because they did not expect a Cananefatian rebellion, but they were aware of the increasing tensions.

Civilis decided on a ruse. He took it upon himself to criticize the commanders for abandoning their forts, and offered to deal with the outbreak of the Cananefates in person with the help of the unit under his command. As for the Roman commanders, they could get back to their respective stations. But the Germans are a nation that loves fighting, and they did not keep the secret for long. Hints of what was afoot gradually leaked out and the truth was revealed: Civilis' advice concealed a trick. Scattered units were more liable to be wiped out, and the ringleader was not Brinno, but Civilis.
Here we meet Tacitus at his most malicious. He does not mention the Roman commander who saw through Civilis' stratagem and investigated what was going on, but it must have been someone higher in the military hierarchy than Civilis - in other words, Marcus Hordeonius Flaccus. In the sequel of Tacitus' story, his description of the defeat of Aquilius, we see how the Romans are reinforced by ships. Guess who was responsible for sending them.