The emperors found themselves obliged to recruit their legions from the frontier provinces, where the genius of war, though languishing, was not totally extinct; and these mercenary forces, careless of laws and civil institutions, established a military government no less dangerous to the sovereign than to the people.
The further progress of the same disorders introduced the bordering barbarians into the service of the Romans; and those fierce nations, having now added discipline to their native bravery, could no longer be restrained by the impotent policy of the emperors, who were accustomed to employ one in the destruction of the others.

Sensible of their own force, and allured by the prospect of so rich a prize, the northern barbarians, in the reign of Arcadius and Honorius, assailed at once all the frontiers of the Roman empire; and having first satiated their avidity by plunder, began to think of fixing a settlement in the wasted provinces.

The more distant barbarians, who occupied the deserted habitations of the former, advanced in their acquisitions, and pressed with their incumbent weight the Roman state, already unequal to the load which it sustained.

Instead of arming the people in their own defence, the emperors recalled all the distant legions, in whom alone they could repose confidence; and collected the whole military force for the defence of the capital and centre of the empire.

The necessity of self-preservation had superseded the ambition of power; and the ancient point of honor, never to contract the limits of the empire, could no longer be attended to in this desperate extremity.
Britain by its situation was removed from the fury of these barbarous incursions; and being also a remote province, not much valued by the Romans, the legions which defended it were carried over to the protection of Italy and Gaul.