Short answers, pulled from the story.
Archaeologist Are Kolberg identified four aspects: the military, the trade, the gift, and the plunder. Each aspect explains a different way Roman objects came into Germanic hands and spread as far north as Scandinavia.
In 9 AD, the Germanic leader Arminius led a revolt against Rome and destroyed three Roman legions in a surprise attack. The Roman commander Publius Quinctilius Varus died in the defeat, and Rome permanently withdrew its frontier to the Rhine.
Roman objects reached Scandinavia through military service, trade networks, diplomatic gifts, and war plunder. Trade routes may have predated the Roman Empire itself, as shown by Scandinavian amber recovered at Mycenae in Greece.
The Illerup Ådal excavation uncovered large quantities of Roman and Roman-inspired arms and equipment, many bearing Scandinavian factory marks modeled on Roman stamps. This shows Germanic peoples both received and independently produced standardized Roman-style military equipment.
The Marcomanni maintained a standing army of seven thousand infantry and four thousand cavalry. Rome sought to cultivate them diplomatically as a counterweight to neighboring Germanic tribes rather than risk open conflict.
Hilaire Belloc and Norman Cantor both argue against the idea of a sudden barbarian conquest. Cantor wrote that Germanic peoples sought to become part of the Roman Empire, fighting for land and money as settlers and allies, and that they permeated Roman civilization across three centuries rather than defeating it in a single cataclysmic battle.