Ultimate X-Men, Volume 5: Ultimate War
The Ultimates vs. the Ultimate X-Men: the battle begins. When the X-Men do the worst thing they could to humanity, the government orders Captain America, Iron Man, Thor and the rest of the Ultimates to bring them down. A small but lethal army, the Ultimates were created to face these and other newly rising threats to mankind. But the X-Men's founder, Professor X, hasn't been training his students for nothing -- and the young mutants just might take out the Ultimates first.
Collecting Ultimate War #1-4.