Take Back the Sky by Greg Bear
Same war. Different enemy.
First it was Mars, then Titan - the battlefield changed but the war remained the same. Until now.
Master Sergeant Michael Venn and his soldiers now know the truth about what the supposedly benevolent Gurus are really doing in our solar system. A truth both Earth and the alien Antagonists are intent on wiping out.
The soldiers must forget their training, forget what they know, and journey to Planet X - infamous home of the Antagonists. Hunted by friend and foe alike and desperate for answers, they will do anything to survive.
Even team up with their greatest enemy.