Roman discovery of Germania began with Caesar's invasion of Germania. In his Gallic Wars, he talked about the River Rhine, the "endless Germania" and the "Germanic fighting machine". One legacy of Caesar is that he clearly defined the boundaries between Gallia and Germania- he defined the Rhine as a boarder, and Germania as the territory to the east. However, the Roman geography east of the river Rhine is hazy, and in Julius Caesar's Bellum Gallia, he described Germania as a piece of land where "there's no boundary"- the "Germania omnis". He described the Germanic people knowing only one business- war.
Caesar tried crossing the Rhine a few times, but was not able to do anything, so he turned his attention back the the Gauls. He said that the Gauls were unable to govern themselves and how it was he, Caesar, who brought peace and order to the people there. He presented the Gauls as peoples waiting to be integrated and "pacified".
Decades later, Tacitus would write his famous book Germania in 98 CE. Although Tacitus himself did not travel to Germania once, he was able to offer a captivating depiction of the people there based solely upon Caesar's description. However, he did slighly move away from Caesar's descirion of Germania being a completely aggressive and war-like state. Tacitus instead depicted the Germanic tribes as lazy. He also described how the Germans possessed "fierce blue eyes, tawny hair, huge bodies", and prized freedom, scorned luxury and esteemed military courage, qualities that centuries later, Adolf Hitler would look to to create the pure, ideal German.
It is worthy to note that the Roman discrouse about the Germanic north was imbued by political interests and that the writers were more concerned with their respective arguments than with Germanic reality. Thus, stereotypes were chosen to the extent that they suit one's overall goal.