Ultimate X-Men, Vol. 1 #5 (2001)

Tomorrow People, Part 5: Killing Fields
Cyclops struggles with the tactics employed by the Brotherhood, but he remains with them. Meanwhile, despite the Professor's misgivings, the United States government launches a full on Sentinel attack on the newly discovered Savage Land. It doesn't take long for Magneto to rewrite their circuitry to attack humans instead of mutants, but it's long enough for heavy mutant casualties. Realizing that the war Xavier fought against is upon them, Cyclops makes contact with Xavier to warn him that Magneto plans to ruin America.
Release Date: 2001