By the early 5th century, the Roman Empire could no longer defend itself against either internal rebellion or the external threat posed by Germanic tribes expanding in Western Europe. This situation and its consequences governed the eventual permanence of Britain's detachment from the rest of the Empire.
In the late 4th century, the empire was controlled by members of a dynasty that included the Emperor Theodosius I. This family retained political power within itself and formed alliances by intermarriage with other dynasties, at the same time engaging in internecine power struggles and fighting off outside contenders (called "usurpers") attempting to replace the ruling dynasty with one of their own. These internal machinations drained the Empire of both military and civilian resources. Many thousands of soldiers were lost in battling attempted coups by figures such as Firmus, Magnus Maximus and Eugenius.
The Empire's historical relationship with Germanic tribes was sometimes hostile, at other times cooperative, but ultimately fatal, as it was unable to prevent those tribes from assuming a dominant role in the relationship. By the early 5th century, as a result of severe losses and depleted tax income, the Western Roman Empire's military forces were dominated by Germanic troops, and Romanised Germans played a significant role in the empire's internal politics. Various Germanic and other tribes beyond the frontiers were able to take advantage of the Empire's weakened state, both to expand into Roman territory and, in some cases, to move their entire populations into lands once considered exclusively Roman, culminating in various successful migrations from 406 onwards. The crossing of the Rhine caused intense fear in Britannia, prone as it was to being cut off from the Empire by raids on the primary communications route from Italy, to Trier to the Channel Coast. In the event, this was much more than just another raid.