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. Chronology 383–388 In 383, the Roman general then assigned to Britain, Magnus Maximus, launched his successful bid for imperial power, crossing to Gaul with his troops. He killed the Western Roman Emperor Gratian and ruled Gaul and Britain as Caesar (i.e., as a "sub-emperor" under Theodosius I). 383 is the last date for any evidence of a Roman presence in the north and west of Britain, perhaps excepting troop assignments at the tower on Holyhead Mountain in Anglesey and at western coastal posts such as Lancaster. These outposts may have lasted into the 390s, but they were a very minor presence, intended primarily to stop attacks and settlement by groups of Déisi, from Ireland. Coins dated later than 383 have been excavated along Hadrian's Wall, suggesting that troops were not stripped from it, as once thought or, if they were, they were quickly returned as soon as Maximus had won his victory in Gaul. In the De Excidio et Conquestu Britanniae, written c. 540, Gildas attributed an exodus of troops and senior administrators from Britain to Maximus, saying that he left not only with all of its troops, but also with all of its armed bands, governors, and the flower of its youth, never to return. Raids by Saxons, Picts, and the Scoti of Ireland had been ongoing in the late 4th century, but these increased in the years after 383. There were also large-scale permanent Irish settlements made along the coasts of Wales under circumstances that remain unclear. Maximus campaigned in Britain against both the Picts and Scoti, with historians differing on whether this was in the year 382 or 384 (i.e., whether the campaign was before or after he became Caesar). Welsh legend relates that before launching his usurpation, Maximus made preparations for an