Following the Nuclear attack from Skynet on August 29, 1997, killing 3 billion people. A human resistance was started. For a better part of a decade. John Connor lead the Resistance against the machines. Sometime during the war, John Connor saved a young Kyle Reese from a Terminator in a sewer. There were many battles.
Two of the most important battles took place on the same day. Operation Chrono and the Colorado Offensive John, knowing that Skynet had a secret weapon they planned to use to help them win the war, organized both assaults. The Colorado Offensive was the mission to shut down Skynet's grid so, Connor and his men could get inside Skynet and see what their weapon is. Skynet uses their new weapon, A time displacement device to send a T-800 back in time to May 12, 1984 to try and kill John Connor's mother, Sarah before he is born. John reveals that he new all this and sends Kyle Reese into the past to protect his mother.