Sub-Roman Britain
Initially, Subroman Britain successfully fought the invasion, thanks to a mythical king called King Arthur.  Probably he was Ambrosius Aurelianus, a powerful Romano-British leader around mid-5th century.  He was renowned for his campaigns against the Saxons, and there is some speculation that he (or his son, according to the historian Gildas) may have commanded the Romano-British forces at the Battle of Badon Hill, where the barbarians were defeated around 500 AD.