Until the 1940s, italians were considered black.
In the south, they couldn't go to white schools, they couldn't marry white people, a lot of the laws applied to them.
They were offered the branch of whiteness, and to fit in, they go hard on blacks because they want to try to erase that connection.
Irish were considered black in the US until the Mexican-American war, they were granted whiteness for their participation and support and they've been treating blacks like hit ever since.
Thats why I don't like the term white supremacy, it sort of makes it seem like whiteness is the goal or white purity.
What it really is, is a system of enforced black-inferiority. This just make sure they keep blacks on the bottom and wave a branch of inclusion to other groups who get on code with them.
HIspanics will be the next group that becomes "white"