> Doing things needs a reason behind it.

Land.

> whose population is and always will be hostile to you?

I don't think they'll really have any problem subjugating and marginalizing populations they don't like. And there's a historical precedent for holding everything up to Berlin.

When the US empire fails, why wouldn't they want it all back?

Again, a hostile population is an issue - but there's things you can do. Look at what the US does to Americans...