I feel like I've said too much in the first few chapters of my novel, but it feels necessary. The story tells of who is invading, and it tells their backstory. Is that too much? Should I wait until they're already invading to say what their intentions are?
You may write your story any way you wish. I'm surprised by so many who have read stories who do not apply that information and experience to their writing.
Stories do not have to be written in a linear way. You can start your story at any point in time. You do not need to front load info-dumps and backstory.
e.g. You could start with Earth troops taking off for the final conflict. As the countdown begins your MC reflects on his personal journey to this point in time. The reflection can run to several pages or chapters.
Roll the story on in real time until Bob asks Steve what made him sign up to 'SpaceForce'.
"London," replies Steve. "They gave us the 'We come in peace bullshit'. . . and we believed them, dropped our guard. They levelled the entire City. I had family in London . . ."
Every time a character has an extended though you are going back in time.
The second novel I ever wrote opens with a pregnant woman sitting in a waiting room she begins to recall the events that got her there - she's still sitting in the same place 300 pages later.
First, let's talk content. If you're revealing all the backstory early on, that means that the backstory - the parts you're telling the reader about - is not supposed to be a mystery. On the contrary - it will help the reader understand your story better. From this standpoint, it's not only ok, it's almost required that you do this, since the rest of the story might not make sense otherwise.
However, there is the second matter, of style. You can share the background with the reader, but there are alot of ways to do this. The simple way is as the all-knowing narrator, just describing the background. A more interesting way is to have it be told by someone - a wise old man who lived through the events, or perhaps an older sister telling her brother a story about the past. Each of these will give you a chance to not only tell the background it will also be a chance to reveal stuff about your characters, the ones telling or hearing the story.
Another advantage of this approach is that the reader sees this as one version of the story, since the story-teller is not all-knowing. Thus, this gives you the option later on to add or change things as you will, and the reader will attribute the contradiction to the specific viewpoint of the story-teller, not to your being inconsistent.
Bottom line - I'd focus my attention here more on the form than on the content, since it seems you are confident about the content being important from the start.