It is widely believed that the Polish-Soviet territorial settlement of late 1920 did not have much to do with ethnic factors. However, this research study shows that they did play a significant role. Both sides established their contrary territorial programs based on their different understandings of the historic Borderlands’ ethnic geography, although political, strategic, and ideological factors were also taken into account. Since neither Piłsudski’s federalism nor Lenin’s Soviet nationalism found much support within the Borderlands, the issue of which program would prevail had to be resolved on the battlefield. The peace negotiations which ended the war reflected to no small degree the concern of both sides for ethnic geography. In the end, the Riga preliminary treaty mirrored the Polish territorial program in its alternative, incorporationist version, which was more moderate than Piłsudski’s federalism. Ultimately, the assessment of whether the Riga border had any justification in ethnic terms depends on one’s view of the Borderlands’ ethnic geography. There is no doubt, however, that ethnic considerations played a significant role in the process of shaping it.