Those who enjoyed watching Gladiator , have a special memory of the opening scene of the film:the battle between the legions commanded by Máximo and the barbarian tribes that oppose them. At one point, these ferocious enemies sing an overwhelming war song that, due to movie magic, has a trick:it is actually a soundtrack taken from Zulu , a film that director Ridley Scott likes a lot. The Zulu chants hit the German mouth perfectly, adding to its impressive appearance. Of course, nothing to do with what some classical sources say about the so-called Groans of Britons , that is, the Groans of the Britons.
Evidently the Britons were barbarians in Roman eyes but not Germans. They inhabited the south of Albion, present-day Great Britain, being of Celtic culture and Brittonic language (one of the Celtic linguistic groups along with the Goidelic of Scots and Gaels). They could not prevent the invasion of Rome, which Julius Caesar carried out twice, in the years 55 and 54 BC, the first probably as recognition and the second to restore his ally Mandubracio to the throne. Later, Augustus planned three other conquest campaigns that did not materialize because he needed the troops in other places or by agreement with the natives. Caligula also had everything ready but in the end he put his soldiers to collect shells and it was Claudius who finally occupied the islands in the year 43 AD
The British leaders Togodumnus and Caratacus resisted although in the end the Roman war machine prevailed. From there began a stage of settlement, pacification and expansion that occasionally broke sporadic rebellions, of which the most famous was the one carried out by Queen Boudica between the years 60 and 61. However, the Romans arrived in Caledonia, present-day Scotland , where Hadrian's Wall was erected as a border because the cost of maintaining control in that region outweighed the benefits.
Of course, that limes it proved relatively effective while the legions guarded it. But when the Roman Empire of the West began to crumble, it dragged with it all its political and military work, all the military resources being necessary to protect the limits of Rome itself. In this context, Picts and Scots, the peoples who inhabited the highlands to the north of the wall, found the opportunity to jump over it and fall on the south. The former were descendants of the Caledonians while the latter originally came from Ireland; both were considered very warlike and used to make rapid raids that, without opposition, spread terror among the Britons.
They were later joined by the mysterious attacotti and to this we had to add the raids that the Saxon Germans and the Franks began from the continent. The chaos caused by the decline of the power of Rome increased with the bands of indigenous Britons and even deserting legionnaires - there were many -, characterizing the scene from the fourth century. Emperor Valentinian named Comes Britanniarum (something like Count of Britannia) to Flavio Teodosio, general and father of the homonymous future emperor -who accompanied him in this episode-, to solve what had been called the Great Conspiracy (because all those enemies seemed to have agreed and in some cases it was).
Flavius Theodosius brought with him German troops made up of Batavians, Heruli, Jovians, and Victors. He landed in Britannia in 368 and shrewdly offered an amnesty to all deserting legionnaires which, when widely accepted, allowed him to replenish the depleted garrisons. He then relentlessly advanced towards Hadrian's Wall pushing the invaders back to the other side, then restoring Roman order, also creating the new province of Valentia as a buffer. He had many mutineers executed but others were incorporated into his army, in the case of the attacotti , using them in subsequent continental campaigns.
The tranquility lasted a little less than a century, after which the evils reproduced again. It was because Constantine III, increasingly in need of troops, withdrew his army from the archipelago in the year 407 and with it his administration, leaving the Britons abandoned to their fate. In 451, what was baptized as Gemitus britannorum reached the ears of Rome. , the Wail of the Britons, a dramatic plea for help from the rulers south of the limes to face that danger. According to the British clergyman Gildas in his work De Excidio et Conquestu Britanniae (On the ruin and conquest of Britain ), written in the second quarter of the 6th century and which constitutes a testimony of the apocalyptic situation that the country was experiencing, the addressee of the message was General Flavio Aetius.
Aetius, veteran of the Battle of the Catalaunian Fields and one of the last remaining supporters of the tottering empire to the point of earning the nickname the last Roman , he was emerging as the most suitable to redirect things. Having repulsed Attila's unstoppable Huns was an unbeatable endorsement, of course; Even if it was thanks to a great coalition, achieving it also had its merits. In reality we do not know if Aetius was really the recipient of the S.O.S, as we also ignore his answer if there was one, since Gildas' text does not mention his name exactly but that of Agitius, and the sources later do not clarify much either because they take his work as a base, such as the Historia ecclesiastica gentis Anglorum (Ecclesiastical History of the People of the Angles) by Bede the Venerable , a Benedictine monk who lived in the following century.
So this Agitius is identified with Aetius, dating the period between the year 446, when he received his third consulship, and 454, when he began his fourth. However, there are authors who believe that Gildas was referring rather to Egidio, another soldier who had distinguished himself in Gaul by confronting the Visigoths and reigning de facto in the north of that province by refusing to recognize the legitimacy of the Emperor Libyan Severus. The problem with this interpretation is that Egidio was never a consul. Now, did things really get so serious?
Some historians say no, that the archaeological record does not indicate it and that Gildas used a hyperbolic tone in his narration. He was a chronicler of superior culture, educated by teachers and not in a monastery, who used a difficult Latin and who, above all, despite being a native of Britain, was deeply Romanized. At that time Rome, although decadent, was still the beacon of the world and of Christianity. His absence means chaos by definition and it is likely that he magnified the news of some conflictive periods that would have reached him by word of mouth. 
In De Excidio et Conquestu Britanniae the Britons are portrayed as too godless and disunited to defend themselves against the threat of Picts and Scots. They only had some success when they put themselves in God's hands but they were punctual moments and the matter was only channeled when King Vortigern hired Angles, Saxons and Jutes as mercenaries, Germanic peoples from Anglia, Saxony and Jutland respectively. What happens is that the newcomers, once the work was finished, decided to stay and create their own kingdoms there, making Vortigern look like the fool who had opened the door for them and was deposed by his own people.
However, Anglo-Saxons had been recruited earlier, after the departure of the legions in the first decade of the 5th century; they had even given rise to some Germanic communities on British soil, as archeology shows. Be that as it may, that wave with its consequent emancipation, which is not clear if it was imposed or mixed with the native British substratum, laid the foundations of what would be the English medieval Heptarchy of the seventh century, formed by the four main kingdoms (Wessex , Mercia, East Anglia and Northumbria) and three minor ones (Sussex, Kent and Essex). From their fusion, seasoned by the Viking contribution, the already unified England of the 10th century arose.
And all for a moan.