The story follows a group of military men. Earth goes to Mars, encounter primitive Martians, the team are in the military forces of Earth which then proceed to kill them all. The end of the book is the campaign on Jupiter, where they wipe out waves of primitive Jovians with atomic guns. The Jovians may have been bird like. One or more of the group, now sick of the slaughter, joins the Jovians, eventually dying along with the last of the aliens when they blow up their remaining stronghold with captured atomics. Although it started with mankind as the force of good, colonialism, xenophobia and genocide showed we clearly weren’t.
They might have invaded Saturn (and wiped out the natives) in between Mars and Jupiter, I can’t quite remember.
I think it was written in the fifties, due to the atomic technology, although it could have been earlier, what with humans running around on the surface of Jupiter, which was just a very big planet rather than gas giant.