foederati

In the late Roman Empire, trusted native tribes which defended coasts or frontiers from further incursions as Imperial authority receded. Foederati were established in Britain in the 4th century AD, for example the Damnonii of southern Scotland who seem to have entered into an alliance with the Romans in the reign of Theodosius. Gildas and Bede record that in the mid-5th century AD Vortigern invited the Anglo-Saxons to settle in Kent to defend the coastline.