From Losurdo's book, War and revolution:

With the emergence of the anti-colonial revolution in the wake of the October Revolution, new anxieties arose or old ones increased. What was happening in the colonies and why were the savages rebelling? Who was inciting them, challenging the healthy, natural racial hierarchy? One thing was certain: a new, deadly danger must be recognized and ‘The Menace of the Under Man’ confronted. Such was the subtitle of a book published in New York in 1922, whose author was Lothrop Stoddard. He spelt out the significance of the term coined by him. It referred to ‘all those melancholy waste-products which each living species excretes’, the mass of ‘inferior’ elements, ‘the unadaptable and the incapable’, ‘savages and barbarians’, who were often full of resentment and hatred for ‘superior’ personalities, who had now proved themselves ‘unconvertible’ and ready to declare ‘war on civilization’. Such was the terrible threat, at once social and ethnic, which absolutely must be averted ‘if our civilization is to be saved from decline and our race from decay’. The book we are referring to was rapidly translated into German: Under Man became Untermensch (in the singular) and Untermenschen (in the plural). This was a keyword of Nazi ideology and Rosenberg acknowledged the US author for having coined it.

Let us take stock. The terminology that began to emerge and become established between the late nineteenth and early twentieth centuries is symptomatic. The main categories and keywords of Nazi ideology – those giving radical expression to its destructive charge against the universal concept of man and its genocidal drive, or which in any event afford a glimpse of the horror of the Third Reich – all go back, directly or indirectly, to the colonial tradition. Konzentratsionlager is a calque of ‘concentration camps’; Untermensch is the literal translation of ‘Under Man’; the Endlösung of the Jewish question recalls the ‘ultimate solution’ of the black question, or the ‘final and complete solution’ of the problem of colonial peoples; the Blutschande (against which Nazism endlessly warned) brings to mind ‘miscegenation’ (a cause for horror in the USA of the white supremacy); behind Rassenhygiene is clearly ‘eugenics’. As regards ‘war of extermination’, ‘destruction of the race’ and ‘holocaust’, comment is superfluous. If the terms in italics presided over Hitler’s attempt to build a racial state in Germany and the German Empire, those in quotation marks date back to the British Empire and, above all, the American one, or the regime of white supremacy that raged against Native Americans and blacks in particular, but which did not spare immigrants variously suspected of being alien to the pure white race.

There is no doubt that the laboratory of the Third Reich and of the horrors of the twentieth century was in full swing; and it went back to the colonial tradition, or the history of the treatment inflicted on the ‘barbarians’ in the colonies and the metropolises themselves by those who proclaimed themselves the exclusive representatives of Civilization.

Accordingly, when historical revisionism and The Black Book of Communism date the start of the history of genocide and horror from Communism, they engage in a colossal repression. Solemnly proclaimed, the moral commitment to give voice to unjustly forgotten victims turns into its opposite – a deadly silence that buries the Native Americans, the Herero, the colonial populations, the ‘barbarians’ for a second time. This is a silence fraught with consequences on a specifically historiographical level as well, because it makes it impossible to understand Nazism and Fascism.

Frequently they blame ""communist"" politicians for letting the immigrants in

Why not both?