Despite the sacrifices of the T-800 Model and Dr. Miles Dyson, Sarah Connor is still considered a fugitive by the authorities and is still on the run. But Judgment Day has merely been delayed, not prevented.
Skynet, the self-aware AI which is created in the near future is still trying to kill John Connor and continues to send more time traveling Terminators after him. If they can kill him before he becomes the leader of the human resistance, Skynet may preempt its own destruction and be unstoppable.
Running is no longer an option. Hiding is impossible. So Sarah Connor declares that the only possibility is to prevent Skynet from ever being created. To change the future and change John's fate. To stop Armageddon before it starts. The war for tomorrow has begun today!