The War Against the Machines broke out following Judgment Day in 1997.
The War[]
A human Resistance was formed sometime after Judgment Day, the nuclear attack initiated by Skynet on August 29, 1997, which killed 3 billion people. For a better part of a decade. John Connor led the Resistance against the machines.
Two of the most important battles took place on the same day in 2029: Colorado Offensive and California Offensive. The Colorado Offensive was aimed to destroyed the Skynet Central Core, while the California Offensive was to stop Skynet's secret weapon — the tactical time weapon — from being used.
Though the Skynet Central Core was destroyed, a Model 101 Infiltrator had been already sent to 1984 with a mission to terminate Sarah Connor, John Connor's Mother. Sergeant Kyle Reese volunteered for the mission. However, upon he was leaving via the Time Displacement Device, Alex attacked Connor and the other Tech-Com members, causing the timeline shift. Hence, Kyle Reese arrived in an alternate timeline, which was created due to the Attempted Assassination of Young Sarah Connor in 1973. Terminator Genisys