Condensed Roman-British and Anglo Saxon Timeline
One way to begin understanding Anglo-Saxon and Medieval English literature -- as well as what it means to both speak or be "English" is remember that the ancient "pre-historical" occupants of the Britain were Celtic and Druidic, and then to think about the three or four principle invasions and occupations of Britain. Each subsequent invasion and occupation layers and mixes an additional culture atop and with those that precede it.
Similar Info in Map Form. A graphic representation of influences by language here.
55 BCE Julius Caesar raids England
43 AD Emperor Claudius occupies England
c. 400 Seeds of Roman Christianity replacing native Druidism
c. 450-476 Collapse of Western Roman Empire: the Romans withdraw from Britain (and the rest of Europe) and leave a power vacuum.
Side Note: The Greek speaking, Eastern Roman Empire thrives as the Byzantine Empire, until Constantinople falls to Muslim Ottoman Turks in 1453.
c. 400 Germanic peoples (Anglo Saxon) settle in Britain
407 Last Roman troops leave Britain with Constantine III
c. 500-15 Possible timeframe for Danish events that occur in Beowulf
c. 515-75 Possible timeframe for Swedish events that occur in Beowulf
597 St Augustine of Canterbury arrives from Rome to convert the English to Christianity
731 Bede completes his Ecclesiastical History
b) Scandinavian/ Viking Occupation 800-1000
793 Vikings attack Lindisfarne
869 Vikings defeat and kill Edmund, king of East Anglia (solidify control of Eastern England)
c. 1000 Viking Leif Ericson discovers North America
Side Note: Mohammed (570 - 632) writes Koran. Islamic religion, culture and "empire" spreads and thrives throughout Middle East, Arabia, and Africa. Muslim "Moors" occupy Iberian Peninsula (Portugal and Spain): c.712-1492. During these so called European "Dark Ages" Greek and Roman learning is kept alive by first Byzantine and then this Islamic culture.
c. 742-814 Charlemagne, beginnings of new European feudal, Christian order (brought to Britain via Normans (see below)
1066 Norman Invasion, under William The Conqueror. End of Anglo Saxon Era and dominance; beginning of Norman or "French" occupation (which essentially lasts until 1453).
A Graphic Representation of Influences: http://upload.wikimedia.org/wikipedia/commons/9/98/Origins_of_English_PieChart.svg