Historians like Peter Heather (Empires and Barbarians) are good on this. For the first 250 years, Rome regularly extended its power across the Rhine and Danube. It negotiated treaties with the German and other groups, on free access for Roman traders (and slave-merchants), access to Roman goods and markets and so on. Bad behaviour was punished by legionary punitive expeditions, cutting off trade, diverting access and favoured treatment to rivals or just inviting the chief to dinner to discuss issues and then killing him. Rome levied drafts for the auxiliaries and took families in and distributed them to provinces short on labour. In other words, all the usual imperial playbook.

Over time the balance shifted. Germania developed, the tribes coalesced into bigger federations, the better to resist Roman pressure (the Alamanni, Franks, Goths, Marcomanni). Roman wars with these were tougher and more expensive, and paid less dividends. The treaties became less one-sided. As civil war, plague, agricultural exhaustion and so on weakened the Empire, the balance shifted. For the last hundred years or so, the Germans were trying to participate in Rome (often as defenders against other groups also wanting a piece), and Rome needed them too much to dictate terms. Then in 350-400 it fell apart under Hunnic and Gothic pressure.