In what I assume is a Cold War allegory, a alien race comes to Earth, tells us that they're in a war with alien race #2. They just need to establish a lookout base and will otherwise leave us alone. Alien race #2 then comes and drives them out, tell Earth that that the #1's lied to us about their real goals and invite us to help in the war effort. #1 then comes back and retakes the Solar System. This cycle continues until the big reveal at the end is that Earth is now burnt hunk of rock and the story is being told by a man huddled around a fire sharing oxygen as he cackles about how "liberated" they are. Title/author, anyone?