Strictly speaking, the Anglo-Saxons never arrived in England since
"Anglo-Saxons" is the generalizing name given later to the invaders
from culturally related Germanic tribes that came to Britain,
including the Angles, Saxons, Frisians and Jutes. Originally, there
was originally no people of that name; they only became the
"Anglo-Saxons" after they had come to Britain. But I will use that
term to make things more concise.
The main problem when trying to assess the reasons that led to the
invasion of Angles, Saxons and other Germanic tribes in the 5th
century is the rareness of written contemporary sources describing the
events and their backgrounds. Therefore, any attempt to irradiate and
explain the historic processes is necessarily in parts speculative.
It was a combination of various historical developments that led to
the Anglo-Saxon invasion of Britain:
In the century before the Anglo-Saxon invasion, the Roman Empire,
espcially its western part, was in a general state of decline.
Internal struggles for power among rival claimants for the Imperial
throne, plagues that depopulated entire regions and cities, and other
factors led into economical, organizational and military demise. The
underguarded provinces of the weakened Empire were more and more often
raided by barbarian tribes from beyond the Empire's borders.
Under these cirmumstances, Saxons were a menace to Romain Britain even
before the invasion. Historian Ammianus Marcellinus (ca 330-395)
already wrote in his history book "Res Gestae" about Saxon sea raiders
who raided Britain in about 365. We can assume that in the late 4th
and early 5th century, the coastal parts of Britain were under the
permanent threat of attacks mainly by Saxons. The documented existence
of a special title for a military commander whose duty it was to
defend the coasts against the Saxon sea raiders - the "comes litoris
Saxonici" - proves this.
The arrival of the Huns in Europe in the year 375 triggered a series
of events and processes that considerably worsened things for the
Roman Empire. Rome not only had to fight the invaders from the east,
but also numerous Germanic tribes that were now fleeing from the Huns,
thus starting what is known as the Migration Period. Support for and
interest in Britain, the most remote Roman province, decreased since
Rome had overwhelming problems in other, more vital parts of the
Between 401 and 410 - an exact date is not known - the Roman garrisons
were withdrawn from Britain, probably because the troops were needed
to defend other parts of the Empire agains invading barbarians such as
the Vandals. From this time, Britain was technically still a Roman
province, but undefended and thus an easy victim for raiders from
overseas or for the Picts (Scots) from the north.
While these developments are still relatively clear, the events that
followed are known through the writings of later authors. Therefore,
it is hard to determine whether their descriptions are basically facts
or already distorted in a legendary way.
According to Saint Gildas' (c. 494 or 516 ? c. 570) history book "De
Excidio Britanniae", the Britons sent a message with a plea for help
against the Pictish raiders to the Roman supreme military commander
Flavius Aetius (390-354) when he had just been appointed consul for
the third time. This happened in 446, so the appeal for military
assistance would date from roughly the same time frame. Aetius, fully
occupied with preparing defense against the threat of a new Hunnish
attack, declined the request.
For Gildas, Aetius' refusal is the reason why the Angles and Saxons
came to Britain. He describes that after Aetius had declined to
provide aid, the British ruler Gurthrigern (or: Vortigern) decided to
hire Saxon mercenaries to fight the Picts. Since it is not clear
whether Gurthrigern/Vortigern is a historical figure at all, it is
hard to say if this fact or myth. Gildas then writes that, after the
Saxons had successfully repelled the Picts, they did not leave Britain
again but began forcefully to subdue the country and let others from
their tribes follow.
Almost two centuries later, Bede (c. 673-735) gives a more detailed
account in his "Historia ecclesiastica gentis Anglorum". He claims
that the events took place in 449 (but this date is disputed today),
and he writes that to fight the Picts, Vortigern hired mercenaries
from the Germanic Saxons, Angles and Jutes, led by the warlords
Hengest and Horsa, who noticed the military weakness of Britain (their
historical existence is not proven). After having defeated the Picts,
the warlords invited more people from their troops' tribes to follow,
then rebelled against Vortigern and began to conquer Britain for
Gildas and Bede are the main sources for what we know about the
Anglo-Saxon invasion of Britain, though they are not absolutely
reliable. Today, it seems probably that by the time these writers
refer to, Angles, Saxons and other members of Germanic tribes had
already arrived in small groups and settled in the coastal regions of
Britain. It appears possible that what Gildas and Bede describe was
historically only the last part of a "slow" invasion over several
decades. Nevertheless, there might well be a factual core in their
descriptions. The main aspect remains unchanged: Members of migrating
Germanic tribes, among them Angles and Saxons, took possession of
Britain because the island had been effectively abandoned by the
weakened, wavering Roman Empire and was virtually undefended by then.
Hope this answers your question!
Britannia.com: The Anglo-Saxon Chronicle - 449-95
Wikipedia: Sub-Roman Britain
Vortigern Studies: Ammianus Marcellinus - Res Gestae Divi Augustae
Wikipedia: History of Anglo-Saxon England
University of Rochester: Vortigern in the Anglo-Saxon Chronicle - Footnotes
Britannia.com: Discussion of the Saxon Invasion of Britain, Part 1
Anglo-Saxons.net: Did they really arrive in AD 449?
Medieval Sourcebook: Gildas (c.504-570): Works
Wikipedia: Ammianus Marcellinus
Britannia.com: Arthurian Britain