Late on Thanksgiving Day, a holiday whose central fable is about the American value of welcoming strangers in need, President Donald Trump announced an intent to “permanently pause migration from all Third World Countries to allow the US system to fully recover.” It is not clear exactly how exactly this sweeping policy is supposed to work in practice — or if, as is often the case, the president’s posts bear little resemblance to policy reality. But it does have a clear precedent: a speech given by Richard Spencer, the leader of the once-prominent “alt-right” movement, given just after Trump’s victory...