> Had nobody stopped Germany I don't see a reason why they wouldn't have succeeded.
Barring complete world domination they would have no chance of doing so, given the existence of Jews beyond Germany's reach (including those who escaped the Holocaust). Yet, we still consider that a systemic mass genocide.
Same deal with the US and its eradication of in some cases entire tribes; just because the US stopped engaging in systemic mass genocide (for whatever reason) or otherwise didn't succeed in killing or sterilizing every last native doesn't mean my country didn't engage in systemic mass genocide at all.
Hell, even by the incorrectly-narrow definition you seem to be using, there are quite a few tribes that are nowadays entirely extinct, e.g. the Karankawa people in Texas and the Yahi people in California.
