In the winter of 367, the Roman garrison on Hadrian's Wall apparently rebelled, and allowed Picts from Caledonia to enter Britannia. Simultaneously, Attacotti, the Scotti from Hibernia, and Saxons from Germania landed in what might have been coordinated and pre-arranged waves on the island's mid-western and southeastern borders, respectively. Franks and Saxons also landed in northern Gaul.

These warbands managed to overwhelm nearly all of the loyal Roman outposts and settlements. The entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.

Nectaridus, the comes maritime tractus (commanding general of the sea coast region), was killed and the Dux Britanniarum, Fullofaudes, was either besieged or captured and the remaining loyal army units stayed garrisoned inside southeastern cities.

The miles areani or local Roman agents that provided intelligence on barbarian movements seem to have betrayed their paymasters for bribes, making the attacks completely unexpected. Deserting soldiers and escaped slaves roamed the countryside and turned to robbery to support themselves. Although the chaos was widespread and initially concerted, the aims of the rebels were simply personal enrichment and they worked as small bands rather than larger armies.