In addition, there was renewed interest - and threats - from the Irish Gaels, Scots and Germanic raiders, all of whom saw rich, undefended booty there for the taking.
Later legend says that Vortigern acted alone, but the near contemporary histories talk about a 'council' supporting him all the way. It would seem that a consortium of high-ranking Romano-Celts panicked and made a decision with major consequences; not least the fact that you're reading this in English.
Vortigern and his council invited two Saxon mercenaries to bring three shiploads of warriors into Britain. They would fight on behalf of the Romano-Celts against the British and all comers. They would be given land to facilitate their needs.
Hengist and Horsa established their bases, and quickly realized the truth of the situation. The Romano-Celts were weak. The British Celts were divided. In short, a united front of Germanic tribes from Jutland, Saxony and Angeln (Schleswig-Holstein today) could probably take it all.
Messages flew back across the North Sea and more ships sailed over the horizon in response. The fighting was fierce (Horsa was killed helping the Jutes take modern day Kent from the Cantii), but with startling speed much of the south of Britain became Angle-land - England.
Thousands of Celtic refugees fled over the Channel into Lesser Britain (Britanny); others stayed and integrated with their Anglo-Saxon overlords.
As for the Romano-Celts, their leaders were all invited to a massive conference in London. It was supposed to be free of all weapons, but the Saxons each had a knife concealed about their person. At a signal, halfway through the welcoming feast, all of the Germanic warriors killed the Romano-Celt beside them. Only Vortigern survived, kept alive for his ransom and his standing as a mouthpiece in future campaigns to take the rest of the Isles.
The white dragon was invited into Britain; and the Celts didn't realize the danger until it was way too late. Nor did the Romans return to save the Romano-Celts.
But half lost in the mists of Celtic lore was an answer to it all. The ravages of the white dragon could be held back, if the red dragon rose to meet it.