Constantine III was declared Emperor by the Roman forces stationed in Britain in 407CE, despite the fact that there was a ruling Roman Emperor of the West, Honorius. Constantine landed in Gaul and established himself at Arles. His son, Constans, withdrew from a monastery to join him. One of Constantine's subordinates, Gerontius, rebelled and in battle defeated and killed Constans. Constantine, however, like Maximus before him, attempted to expand his realm over the Roman West unsuccessfully. He was defeated and surrendered with another of his sons to the forces of Honorius, and both were executed in the summer of 411.
   Both this Constantine and Constantine the Great had sons named "Constans". However, it is Constantine III that is said by Geoffrey of Monmouth and later sources to have been the father of Uther Pendragon and the grandfather of Arthur. In Welsh genealogies, Constantine is given a father called Kynnvor.