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 - written by Mark Millar and illustrated by Chris Bachalo.