Short answers, pulled from the story.
Historiography traditionally dates the Migration Period from AD 375, the invasion of Europe by the Huns from Asia, to 568, the Lombards' conquest of Italy. A more loosely set period extends from as early as 300 to as late as 800.
The Migration Period, dated about 300 to 600 AD and also known as the Barbarian Invasions, was an era marked by large-scale migrations that saw the fall of the Western Roman Empire and the settlement of its former territories by various tribes. Post-Roman kingdoms were established across those lands.
The Migration Period involved the Burgundians, Vandals, Goths, Alemanni, Alans, Huns, early Slavs, Pannonian Avars, Bulgars, and Magyars. Germanic groups such as the Visigoths, Ostrogoths, Anglo-Saxons, Lombards, Suebi, Frisii, Jutes, Sciri, and Franks were among the first to move.
Total migration into Roman territory during the Migration Period could have ranged from roughly 500,000 to over 1,000,000 people. The Gothic crossing of the Danube in 376 is estimated at 90,000 to 200,000 people, and the Rhine crossing of 406 at up to 200,000.
French and Italian historiography uses terms translating to barbarian invasions, such as Invasions barbares and Invasioni barbariche, viewing the events as a catastrophe and the start of a Dark Age. German and Slavic scholars instead speak of migration, using the German word Völkerwanderung, the wandering of peoples.
Historians have proposed climate change, population pressure, and a domino effect from the construction of the Great Wall of China that pushed the Huns onto the Goths. The scholar Guy Halsall argued the barbarian movement was the result of Rome's fall rather than its cause, since the tribes were settled agriculturalists drawn into the politics of an already failing empire.