“Why don’t they go back and help fix the totally broken and crime infested places from which they came?” Donald Trump wondered, of four “progressive” (his “punctuation”) Democrat congresswomen: Alexandria Ocasio-Cortez, born in New York; Rashida Tlaib, born in Michigan; Ayanna Pressley, born in Ohio; and Ilhan Omar, born in Somalia, raised in the US and a naturalised American.

Impossible to tell why he chose these women rather than, say, Mike Pence, also born in the US but of Irish ancestry. Or, for that matter, why Trump himself doesn’t return to the Outer Hebrides, birthplace of his mother, which is riddled with crime (against knitwear). Imagine how incredibly full Scottish politics would be if everyone whose parents were born there went back and tried to govern it. We’d be seeking independence from them.

If it sounds like it makes no sense, it’s because you’re not trying hard enough to understand Trump’s thought architecture. These are basically the politics of Herod: everyone go back to where you came from, then at least we’ll know where we are. Nobody here intends to kill any babies later on. Oh no. Or there’s a simpler reason: straight race hate. American commentators are asking, “Should we be saying ‘Trump has been racist’, or to be precise, ‘Trump has been racist again’?” In fact, the shock factor of an openly bigoted leader on the world stage really belongs to another age. We have known for a long time that Trump distinguishes between races in essentialist, nonsensical ways, that somehow always seem to work out better for you if you’re white – whereupon you’re most likely not a rapist, not a criminal, not a terrorist.

What emerges thematically across his presidency, from Congress to the nation’s borders, is an increasing level of hatred. The “birtherism” that was malevolent but mischievous when Trump levelled it at President Obama has hardened into something far more violent: a question mark over the very legitimacy of Americans who are not white. Never mind could AOC be president: is it moral of her to live in America at all? If it makes no logical sense, from a man with an immigrant wife, an immigrant ex, an immigrant mother and immigrant grandparents, that’s because it’s not intended to: the aim is not consistency, it is the creation of racial hierarchy.

Play Video 1:11 Donald Trump says tweets about congresswomen were 'not at all' racist – video

Close observers of the far right – its obsessions, its deliberate falsehoods, its hot button issues – have noted for some time that race always supersedes nationalism, eventually. As the House speaker, Nancy Pelosi, said, “‘Make America great again’ really means ‘Make America white again’.” Partly underpinning this is a new fantasy of the US as a white-settler state, seeking ever greater purity itself, in readiness for its role as a homeland for white people fleeing Muslim Europe. This is at the root of Trump’s fixation with the London mayor, Sadiq Khan: the president’s insistence – and that of his supporters – that the British capital is full of no-go areas (where, according to one comically lurid Twitter imagination over the weekend, “kebab shop owners will murder and rape you, and feed you to their customers”).

Other myths attach to other parts of Europe – Chancellor Merkel’s refugee policy in Germany generating thousands of rapes in a single night, for instance – but it has become a distinctive and now well-worn fantasy, even for the mainstream US right, to conceive of London as not a multicultural city but an invaded one, whose mayor was voted in by Muslims, whose Subway sandwich bars are all halal. Because it is nonsense, whatever its source, it floats through the discourse like astrology. Just because there are people who believe that rubbish doesn’t mean it need detain the rest of us.

If there is an 'immigrant' who has failed to integrate in America, it's Donald Trump | Richard Wolffe Read more

Yet the real agenda of this myth-making unfolds plainly before our eyes: Trump and his supporters manufacture hostility as the natural state between Muslims and non-Muslims, people of colour and white people, as milestones towards creating a country for white people, where everyone else goes “home”. This, even if we had no evidence of inhumane behaviour towards migrants, no intimation of Islamophobia at a policy level, would be alarming. More alarming still is that what sounds like hyper-nationalism is actually deeply international, relying heavily on global allies.

Donald Trump needs Boris Johnson to gnash his teeth about the areas of Britain where English is a second language; he needs Piers Morgan calling Khan a “pipsqueak” who lacks respect; he needs Nigel Farage to blame Khan for the London Bridge terror attack. It’s not that the US president is particularly interested in what occurs here, but getting racial purity back on to the political agenda requires a huge amount of perceived threat; more than one man could ever manufacture, or even one nation. To revivify this zombie idea (“You don’t belong here because of your skin colour”; was there ever an idea so long-dead, so decomposed?), white supremacists have to position their entire civilisation as at risk. We know figures within our own right wing collude with the narrative; we don’t yet know where their collusion will end.

• Zoe Williams is a Guardian columnist