Although successful in stopping invasion from abroad, Vortigern was forced to fight against his own countrymen at the Battle of Wallop.
According the British chronicler Nennius, when Vortigern rose to power after the fall of Roman rule in Britain, he faced three main threats: The Picts and the Irish, Roman Invasion, and a man named Ambrosius.
In truth, there was no real threat of Roman Invasion. The Empire was well into its death throes and would never again come close to invading the far flung island of Britain. Meanwhile, Vortigern’s successful campaigns against the Irish and the Picts (detailed in early articles) had eliminated that danger. Thus, Vortigern’s one remaining threat came from a man named Ambrosius.
Almost nothing is known about Ambrosius, except a few hints from the chroniclers and guesses based on his name and time-period. His name is Roman, so it is likely the he (like Vortigern) was a ranking nobleman in the last days of Roman rule. Considering the time, it is extremely unlikely that he is the same figure as Ambrosius Aurelianus who would be so important to the British in the 460s, though the two men may have been father and son. All that can be said for sure is that Ambrosius was Vortigern’s main political opponent.
During this time, two central issues dominated British politics: the Pelgian heresy and the upkeep of the Saxon mercenaries. Vortigern brought over the Saxons under Hengest and needed their support to maintain peace. Vortigern seems to have been non-committal when it came to religion. It is thus reasonable to suppose that Ambrosius argued against the Saxons and stood strongly for religion (although whether for Roman Catholicism or Pelgianism can only be guessed). Whatever the exact causes, the political situation became so bad that the two sides met in open warfare in 437AD at Wallop in Hampshire. It is unclear who fought in this battle, certainly Ambrosius and Vortigern and possibly another noble named Vitalinus (though this may in-fact have been the true name of Vortigern). The armies of both sides were likely British, though Ambrosius may have had help from the continent and Vortigern may have been supported by the Saxons.
No victor is recorded for this battle, but considering that Vortigern remains in control after 437, his victory can be assumed. However, the battle likely cost Britain much of its remaining fighting power, and possibly strengthened the internal dissentions within Britain. As a result of the battle of Wallop, Vortigern was forced to lean even more heavily on his Saxon mercenaries, and gave Hengest permission to bring even more Saxons to Britain. It was a move that would cost him everything.
