Gildas, whose main objective was to castigate his countrymen for their lack of religious faith, describes  a council which debated how best to repel invasions from the Picts and Scots:

And they convened a council to decide the best and soundest way to counter the brutal and repeated invasions and plunderings by the peoples I have mentioned. [Picts and Scots, referenced in earlier passages.] Then all the members of the council, together with the proud tyrant, were struck blind; the guard (or rather the method of destruction) they devised for our land was that the ferocious Saxons (name not to be spoken!), hated by man and God, should be let into the island likes wolves into the fold, to beat back the peoples of the North.

Bede, writing two centuries later and relying on Gildas (as well as other sources), notes the attacks were harsh, and frequent, and the Britons were apparently powerless to stop the incursions:

They [the Britons] consulted as to what they should do and where they should seek help to prevent or repel the fierce and very frequent attacks of the Northern nations; all, including their king  Vortigern, agreed that they should call the Saxons to their aid across the seas.

Once the Saxons (a name used by Britons to describe all the various invading tribes)  answered the call,  it was too late to rescind the invitation. Gildas describes  their arrival:

Then a pack of cubs burst forth from the lair of the barbarian lioness, coming in three keels, as they call warships in their language. ... On the orders of the ill-fated tyrant, they first fixed their dreadful claws on the east side of the island, ostensibly to fight for our country, in fact to fight against it.

When the “wolves” are in the fold, it’s pretty difficult to make them leave.