Originally Posted by Twistys Tim

Biggest problem with multiple timelines / universes idea is this; In the future, SkyNet would have known that sending the machine back in time to kill Sarah Conner would not effect change on it's current predicament. SkyNet would have understood that killing Sarah Conner in the past would not kill John Conner in the present according to the multiple timeline theory -- and a machine as powerful as SkyNet (which could invent a time machine) would certainly understand that the fact that if it was defeated by humans in the present, it would means that it could not change that fact by attempting to change the past.