Well, to be clear, that’s not going to happen because faster than light travel is impossible. But if aliens did manage to make it here then it would be hard for them to have anywhere near the fire power to enslave every single human on Earth. There are billions of humans on Earth. Humans would have the numeric superiority to any visiting aliens. Plus we have a lot of fire power ourselves.
In short, it would be impossible for them to enslave every single human, and for what purpose? What would they gain from it?