Cartimandua has seized and holds hostage Venutius' brother and other relatives, Venutius makes war against her in 57, then wars against her Roman protectors.

Building alliances outside the Brigantes, he stages an invasion of the kingdom.

The Romans, having anticipated this, send some cohorts to defend their client queen.

The fighting is inconclusive until Caesius Nasica arrives with a legion, the IX Hispana, and defeats the rebels.

Thanks to this prompt military support from Roman forces, Cartimandua retains her throne.

Tacitus presents Cartimandua in a negative light in his moralizing narratives, the Annals and the Histories.

Although he refers to her loyalty to Rome, he invites the reader to judge her "treacherous" role in the capture of Caratacus, who had sought her protection; her "self-indulgence" (her sexual impropriety in rejecting her husband in favor of a common soldier); and her "cunning strategems" in taking Venutius' relatives hostage.

However, he also consistently names her as a queen (regina), the only one such known in early Roman Britain.

Boudica, the only other female British leader of the period, is not described in these terms.