400 CE. The Roman Empire, weakened by wars of succession and barbarian onslaughts, has split into the East and the West. The East, headquartered in Constantinople, survives for a thousand years as the Byzantine Empire. Here Greek prevails and Christianity, institutionalized as the Orthodox Church, spreads into Slavic, Eastern Europe. The West, headquartered in Rome, quickly disintegrates under Attila the Hun, the Goths, the Vandals, and epidemics. Here Latin prevails and Christianity, institutionalized as the Catholic Church, spreads into Germanic, Northern Europe... Rome abandons Celtic Britain; Angles and Saxons conquer it.
800 CE. Charlemagne establishes the Holy Roman Empire centered in Germany. Although "neither holy, nor Roman, nor an empire", it offers Europe some political structure during the Middle Ages and is destined to last a thousand years.
1066 CE. The Norman Conquest of England. The Normans ("Norsemen", Frenchified Vikings), will eventually mix with the conquered Anglo-Saxons to create modern English culture.
Edward Gibbon's Decline and Fall of the Roman Empire