Because this is fun, I will keep hammering at it for my own amusement:
The future does change due to the TX’s actions in T3. It kills several people who were supposed to be Connor’s lieutenants in the future war. The fact that the humans do not tell T800-101 #3 to save these people indicates that the people have already been removed from the timestream and the humans know they cannot be saved (they are also somewhat realistic about the T800’s overall chances against the TX and program into it a healthy caution about just how weak it is compared to the newer model).
The “no fate but what we make” theory of the second film is correct. The problem is, the humans just don’t have the kind of focused plan that Skynet does. They only react to Skynet and are never proactive in trying to stop it. So long as Skynet keeps making contingency plans that compensate for mission failures, they can’t stop it.
Basically, there is no fate but what Skynet makes.
Skynet plans all of its terminator time travel missions simultaneously – years before it actually has a time machine built. It calculates that if human resistance does manage to make headway, it will be due to one or more powerful figureheads. Thus, it will selectively target them for temporal assassination. It continually analyzes the timestream to pick up the unsubtle clues as to the success or failure of the missions (17 police officers killed in an LAPD precinct in 1984, Cyberdyne systems up in flames in 1991, etc.)
For example, Skynet, after sending mission 2, knows Cyberdyne was blown up in 1991 and that it was born in 2003. It estimates that Cyberdyne’s research would have pushed its birth back to 1997, had it been able to continue. It knows, somehow, that John Connor is still alive. It knows that a human probably couldn’t have defeated the T1000, so a T800 must have been sent. It knows that the T800s near the time machine are the most likely to be/have been sent. It tells all the T800s in the lab that Cyberdyne/Dyson was solely responsible for its creation, because the only way it can be sure Sarah and John will not try to kill off its incarnation within the Defense Department’s secret projects labs is if they think it’s already dead. It also tells the T800s that Judgement day was in 1997, even though, by that point, it knows it was actually in 2003.
The order in which time travel missions (that we know of)get “sent” is not chronological, obviously:
1 - T800-101 #1 Sent. Termination mission fails, but “tech seeding” succeeds.
2 - T1000 Sent. Termination mission fails. Dyson contingency plan succeeds in making John and Sarah believe that Skynet will never exist, so they do not go any further in trying to make sure.
3 - TX Sent. Tertiary termination mission (lieutenants) succeeds, at least partially. Secondary mission of informing Skynet of the war’s progress succeeds. Primary mission of killing John Connor fails. Skynet is born. Judgement day happens. This essentially maintains the status quo, though the loss of the lieutenants may hinder the human war effort.
4 - Kyle Reese Sent. He succeeds in fathering John and ensuring the survival of Sarah Connor.
5 - T800-101 #2 Sent. It succeeds in protecting John Connor against the T1000, but also delivers misinformation that throws Sarah and John off of Skynet’s scent, but still delays Judgement Day.
6 - T800-101 #3 Sent (Probably occurs at a considerably later date than the other five missions.) Succeeds in keeping John and Kate alive, which is all he was supposed to do.
Basically, Skynet sabotages human missions 4 and 5 based on its analysis of mission 2. It does not have time to do anything about mission 6, because the humans presumably capture the time machine very shortly after mission 3 is sent (and the mission log of the machine is likely wiped, partially explaining why Reese does not know about the T1000 or TX missions).
Now the humans get ahold of the time machine, and some of Skynet’s records. They confirm with Connor that Skynet sent a T800 back to 1984 (one is missing from the rack, according to the novelization of T2 – take it for what you will). John knows he must send Reese back, so he does. However, some confusion over some of Skynet’s information leads other members of Connor’s team to believe that Mission 4 will result in Judgment Day moving forward to 1997, because Cyberdyne will get a leg up via the T800 processor (could be true, but isn’t). Reese is either confused by this information, or believes it, so he tells Sarah the wrong date in 1984. Connor is not privvy to this discussion because he is busy trying to reprogram T800 #2 while looking nervously over his shoulder for T800 #3.
Connor saw this inactive T800 on the rack, which he remembered as the one who protected him in the past. He reprograms its mission and sends it back to 1991 to do just that.
The humans (except for Connor and Brewster) do not know about the TX mission, as Skynet has concealed it completely except from a few remaining loyal T800s.
“Wait,” says the person bored enough to read this post, “We know the time machine can send out of order, because the missions arrive downstream in the order 1, 4, 2, 5, 3, 6, so why don’t the humans reprogram another T800 (or several) to go on missions 4, 5, and 6?”
Mission 4: The humans know the first Terminator failed and they don’t want to monkey with what seems to have worked out okay for them. If they sent through more backup, Reese might live, leading to Sarah not becoming as tough as she did, leading to John not becoming the leader he is. They decide, sadly, to let it lie.
Mission 5: They only have one T800 that they can send against the T1000 (which they only know as much about as the T800 tells them–enough to know a human has no chance against it). They probably (logically, but apparently wrongly) fear that Skynet may have rigged the time travel lab with some sort of sabotage, so they send the T800 through immediately, without backup, to 1991, hoping they can prepare more backup and send it through. They never do, however, presumably because T800-101 #3 shows up and kills Connor.
Mission 6: The humans learn about the TX mission from Brewster (Connor and Brewster presumably did not tell them about it before because they were loathe to admit that they were not going to try to stop Judgement Day, because they knew it was probably futile and they didn’t want to jeopardize their own survival. It takes them a while to reprogram that T800 without Connor, but they are very thorough in detailing its mission (pretty much “converting” it, rather than just changing its mission, if its actions in 2003 are any indication) and eventually send it through.
So what’s the real story, here? My theory:
[spoiler]Skynet only really tries to kill Connor twice. 1984 is mostly a failure except for the tech boost to Cyberdyne. 1991 is a failure that is nearly catastrophic for Skynet, if not for its post-game feint. 2003 isn’t primarily an attempt on Connor, but rather a “midwifing” of Skynet’s own birth by the TX. Skynet probably didn’t think the TX had much of a chance of even locating Connor, since he was totally off the map, but thought she might be able to hamstring the rebellion by killing off his officers.
It’s pretty obvious by what happens in T3 that Skynet didn’t freak out because humans were trying to pull the plug, as stated by T800 #2 in T2 and Reese in T1. Thanks to the TX, Skynet already knew they would try, and never even gave them the opportunity. All that stuff T800 #2 says in T2 about Skynet causing the stealth bombers to fly with a perfect safety record is probably spurious info fed into the T800s by Skynet itself just before the humans took over the time travel lab, because Skynet is clearly not exposed to the outside world until late in T3.
The main wildcards in this series are what the future John Connor knows and what he tells people. It has to be remembered, though, that he has been fighting a war for decades since Judgement Day and has probably been becoming more and more paranoid as the day of his death approaches. He also can’t be sure that Skynet only sent three terminator missions into the past (nor can we, really), so he’s not able to serenely outmaneuver Skynet because of his foreknowledge. Quite the opposite, in fact. He only has a limited amount of time to sort out all of this confusing time meddling before he gets killed, and he knows it. Thus, his execution of the counterstrikes may be a little flawed.
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