There is a popular song of Dafydd Iwan "Yma O Hyd" (still here)
that praises the continuity of the Welsh as a people since the
time of Macsen Wledig in the fifth century Macsen is the Welsh
name for Magnus Maximus, who may have been the Dux Britanniarum
(the Roman leader of Britain) and whose military campaigns on
the continent gave him control of the greater part of the
Western Empire until he was defeated by Theodosius. After Magnus
had taken the bigger part of the Roman British garrison to
Europe with him, Britain lay undefended against the Irish
invasions to the west and the Germanic tribes to the east.
Within a hundred years after 410, when the Emperor Honorius had
no troops to spare for the British garrisons, the island was
completely estranged from Roman control, and it was during these
years that the Welsh nation came into being. After the
withdrawal of the Roman legions, Britain found itself being
gradually divided into three areas that later became Germanic
England, Brythonic Wales and Gaelic Scotla