I don't what anyone to take this the wrong way but WWII and the death camps prevented eugenics from becoming a US and British policy.
Once they were hit in the face with the unvarnished truth about where their current path was leading they, to their credit, abandon it.
But the evil lives on.
That's true, before 1945, eugenics was advocated right across the political spectrum (yes, even by conservatives - Winston Churhill was one), but like you said, the Holocaust brought that crashing to the ground - thank G-d.