Julius Civilis
Julius Civilis was a Roman citizen and a member of the royal family that had once ruled the Batavians. Later, the constitution had changed and they now had a summus magistratus ('highest magistrate'), but the family of Civilis was still very important and influential. He had fought in one of the Batavian auxiliary units in the Roman army during the invasion of Britain, and was still commanding a unit. Tacitus calls him 'unusually intelligent for a barbarian', which is a commonplace that Roman authors used to describe non-Romans who had surprised them (e.g., the Roman author Velleius Paterculus uses more or less the same words to describe Arminius, who had defeated the Romans in 9 CE in the Teutoburger forest.)
Julius Civilis and his brother Claudius Paulus -again a name that shows that the man possessed the Roman citizenship- had been arrested in 68 on a charge of treason. According to Tacitus, the charge was trumped up. We do not know the precise nature of the accusation, but we do know the result: Paulus was executed and Civilis was pardoned when Galba became emperor. In the last weeks of 68, Civilis had returned to Germania Inferior, where he was again arrested, and brought to the new governor, Vitellius. This time, there is no reason to doubt that Civilis had been up to something; however, Vitellius had pardoned him as a gesture towards the Batavians. In this way, he hoped to gain the support of their eight auxiliary units. A few weeks later the soldiers indeed sided with Vitellius, and they took part in the march on Rome.

The Banquet
The banquet in the sacred grove illustrates that the Batavians were only partially romanized - or Tacitus wants us to believe this. Otherwise, they would have gathered in a town hall. Tacitus' words remind one of what he writes in his Origins and customs of the Germans.

It is at their feasts that the Germans generally consult [...], for they think that at no time is the mind more open to simplicity of purpose or more warmed to noble aspirations. A race without either natural or acquired cunning, they disclose their hidden thoughts in the freedom of the festivity. Thus the sentiments of all having been discovered and laid bare, the discussion is renewed on the following day, and from each occasion its own peculiar advantage is derived. They deliberate when they have no power to dissemble; they resolve when error is impossible.

This description of the German way of consultation is highly suspect. Like all Greek and Roman authors, Tacitus was obsessed with the opposition between civilization and barbarism. The Romans and Greeks considered themselves to be civilized, and because they lived in the center of the earth's disk, it could reasonably be assumed that only savages dwelt on the edges of the earth. Since they themselves lived on river plains, it was quite obvious that barbarians dwelt in the mountains and forests. (See below; Tacitus even describes the Dutch coast as rocky; Annals 2.23.3) This explains why the Romans and Greeks always mention forests, even when there were no forests at all. As a matter of fact, pollen research has shown that the Dutch river country were hardly wooded in the Roman age. This does not mean that there never was a banquet, but that we must be cautious. Tacitus wants to show that the Batavians were noble savages, he is not necessarily telling the truth.
Another feature of ancient descriptions of far-away people, is that they often resemble each other - after all, they were all living on the edge of the earth. The custom of making a double judgment -one when drunk, one when sober- is also known from another source, the Histories of the Greek researcher Herodotus of Halicarnassus (1.133), who correctly says that it is a Persian custom. Again, this does not mean that the Germans did not consult each other in a state of inebriety, but it warns us that we must remain careful when we read the extremely tendentious Histories of Tacitus.