The film takes place in the near future, where an alien race has invaded the Earth and defeated the world's military units.
It only says that aliens (mimics) invaded Earth. Why did they come to Earth? Who are they? Why do they want war? What do they want from Earth? Are they only soldier aliens, or is there an entire alien race on Earth? None of this is explained in the movie, unless I missed it.
I haven't read the book All You Need Is Kill, and these questions are not answered in the wiki page of All You Need Is Kill. Where, if anywhere, is the origin of the mimics detailed? Where did they come from? And why?