His reason for joining Terra Prime were that he blamed Vulcans for not intervening during World War Three. He said it suited the Vulcans' purposes to wait until Earth was devastated, and thus make the Human race easier to control.
When Paxton questioned Mercer's loyalties, he sent for Greaves, and Mercer was later found dead. Greaves accompanied Paxton to Mars and assisted him in his attempt use the verteron array as a weapon against Starfleet Command. When Commander Tucker attempted to shut down the array, he was shot and wounded by Greaves. Moments later, Greaves was himself felled by multiple phase-pistol blasts from Archer, Reed and Mayweather. He was presumably stunned and taken into custody. (ENT: "Demons", "Terra Prime")