I read this novel at least 25 years ago, maybe more. I don't remember the cover.
There is a joke: How does an elephant come down from a tree ? It sits on a leaf and waits for autumn.
Well, this is how this book begins, by an army of elephant-looking aliens reaching Earth on gliders. And soon they control most of it.
One thing that I remember is that with them everything went by three (or maybe by four ?): their trunks that ended in many delicate "fingers", 9 or more (16?) ; their armies that were organized in a hierarchy by 3 or 4; and so on.
I don't remember much except that those who on Earth tried to keep fighting managed to leave the surface by a rather brutal method: huge (and very thick) metallic plates propelled by exploding nuclear bombs on their down side. (I don't remember how they manage to explode nuclear bombs progressively rather than all the bombs at once).
I think that by that method, once the "elephants" lose full control of the sky, mankind manages to win the war.