The only reason the huge scale of the holocaust happened is because hitler had the best army navy and air force in Europe. So there wasn't lot of people that could say no to him even England Winston Churchill appeased Hitler which means he knew that he was bad but didnt want to start a war yet. But now there is NATO and the UN and multiple super power countries which is hard to say which one is better to pull that off and not be pressured to stop what you were doing. There were little people who tried to stop Hitler before the mass Genocide. But today on the same scale as what hitler did would almost be impossible to do without being stoped.
Mountains and Rivers are obstacles in which are hard to move around. also the other answers make little sense since Native Americans still fished coastlines, hot weather doesn’t do much, and climates suited to farming did not lead them to be a complex civilization
