Single timeline

Everything is reset upon Jane Connor was born in Endgame

In the theory of single timeline, there is only one timeline despite time travelling events, which only makes the timeline constantly shift to accommodate events. If something is changed in the past, the situation in the future will be changed immediately to reflect this new history. Apparently, Skynet believes in this theory, hence it sends a Terminator to 1984 in hope to kill John Connor and reshape the future by terminating his mother, Sarah Connor.

Single timelines in Terminator

The Terminator: Endgame

When Jane Connor was born, the timeline was immediately reset as the "Machine" suddenly ceased to exist in the middle of the fight, while people killed by it revived instantly and were unaware of what was just happened.

RoboCop versus The Terminator

Whenever a crucial event happened in the past due to a temporal interference, the future was immediately changed. The first change happened when Flo Langer shot Alex Murphy. The second change happened when Officer Murphy committed suicide after he had learned he would make Skynet become self-aware and even initiate Judgment Day. The third change happened when the Future RoboCop travelled back to the present day and destroyed the satellite that housed a primitive Skynet. Future RoboCop then ceased to exist, while Flo, who was recently killed, woke up in a peaceful future where Judgment Day never happened.

The Terminator: Rampage

Before its total destruction, Skynet performed a core dump and transferred its vital data into the Meta-Node, a modified T-800. The Meta-Node travelled to 1984 and later took over Cyberdyne Systems Laboratory. This made every human in the future immediately vanish except for the few who stayed in the null-time field, including John Connor and a Resistance commando who later travelled to 1988 to terminate the Meta-Node.

The Terminator: Future Shock

Let's Play Future Shock, Part 40 - Terminator Salvation (End Game)

In 2015, a moment after a Resistance fighter loaded a disk, which contained a virus created by Dr Bill Hanover into Skynet's mainframe, and the virus was sent to the past to corrupt Skynet via Temporal Data Transmitting System, there was a massive time distortion that ripped through the world and instantly transformed Los Angeles from a hellish nightmare into a gleaming paradise.

Terminator Salvation

In 2018, John Connor believed that Skynet planned to kill Kyle Reese, Connor's father, in order to erase John Connor from existence and reset the Future.

In 2029, as Thomas Parnell planned to wipe out Skynet while massacring the Resistance, Skynet sought Connor's help to fight against Parnell. Resistance fighter Simon, Connor's lieutenant, travelled to 2003 with a task to terminate Parnell. Since Parnells' Terminator force continued to attack the Resistance, Connor concluded that Simon's mission had failed because if he had succeeded in his mission, the change in the future would have been instantaneous.

Terminator Genisys

In 2017, John Connor asked Sarah Connor and Kyle Reese to join his side and align with Alex, the Skynet from an alternate future. John stated both Sarah and Kyle would die if they refused. Sarah claimed John could not kill them as they were his parents and John would never be born if they died. However, John dismissed the claim and stated "there truly is no fate".

Notes

• There is a notable example of a predestination paradox causality loop in The Terminator if the audience does not assume the existence of an "unaltered original" timeline: if the original Terminator was not destroyed in the Cyberdyne factory, Skynet, Judgement Day, or the Future War may never have occurred (At least the way the franchise under Cameron is familiar with); whereas if the Tech-Com army and the Skynet army had never sent soldiers back in time, John Connor and Skynet may never have existed, or, John's father may have been someone else instead of Kyle Reese.
• Other examples of single timeline theory outsides Terminator includes Back to the Future and the 2000 film Frequency.

See also

• Predestination paradox
• Alternate timeline - a concept contrast with the single timeline