This naturally put them at odds with the Slavs, especially if we’re also considering Russia’s Pan-Slavic policies at that time. In the 19th century, there was support in nationalist German circles to revive the drive to the East (Drang Nach Osten).
As a matter of fact, the original Prussians were an indigenous Baltic tribe conquered and eventually assimilated by the Teutonic Order. In some discourse, the Drang Nach Osten also included the military expedition and colonization by the Teutonic Order against the Baltic Sea pagans. This migration had a large influence in Eastern Europe, causing legal, cultural, economic, and religious changes. If i have to take a guess, it probably dates back to the Ostsiedlung: the migration and settlement of ethnic Germans in Eastern Europe (inhabited by Baltic and Slavic peoples) during the High Medieval period.