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.