It's funny, but despite really liking the "Terminator" movies (well, actually, I really like the first and third. I think the second his highly overrated...) I never did get around to seeing "Terminator: Salvation." And honestly, I don't think I will, because to me, a "Terminator" film that shows the actual war against Skynet is like a "Star Wars" movie that shows how Anakin Skywalker became Darth Vader. The original films craft just enough of a story to give you an idea of how it happened, while letting you fill in the blanks with your own imagination. The result is a seamless illusion of a greater story that's just as amazing as you imagine it could be, because you are imagining it. You just don't realize it.
It's fun to look back at the Terminator films and try to extrapolate a version of the future from them. For example, we can assume that there was an original, unaltered version of the timeline, because we see that history changes from one movie to the next. Since the time travel doesn't cause a closed loop, then there must have been a "genesis" timeline, one that propagated the changes that led to the temporal war we see. Presumably, in this version, Reese came back in time to accomplish another, unrelated goal--perhaps to sabotage Skynet's systems in some way--and wound up falling in love with Sarah Connor and impregnating her with John Connor.
Presumably, John Connor's existence was a massive turning point for the war. It must have been, because we see from Reese's dreams that Terminators are common enough that humans have standard counter-measures for them, and yet Skynet only sends one T-800 back in time to stop John Connor. Since there's no reason not to send more, it stands to reason that they only sent one because that's all they could send. This implies that the time travel facilities were captured not long after the T-800 was sent back. (You could theoretically ask why Reese was the only human soldier to be sent back, but don't forget, Skynet was trying to change history and the humans were trying to preserve it. They have an incentive to keep disruptions to a minimum.)
Obviously, Reese succeeds in preserving Sarah (and John) Connor's existence. But he fails in one key aspect--he allows the T-800's remains to survive. This gives the designers of Skynet a boost, which explains why they send back a T-1000 instead of a T-800 in the second movie; they've managed to survive a bit longer in the war in the revised timeline, long enough to send back two Terminators instead of one (and one of them an advanced prototype, at that.)
It seems like the heroes win a big victory in the second movie--after all, they destroy much of the research conducted on Skynet, setting back the robot holocaust by years. (If T3 is to be believed, the nuclear war was originally "scheduled" for just after the T1000 arrived.) But the machines did achieve one important goal--they gave John Connor an affection for Terminator models that resemble the governor of California. This allowed them to assassinate John before he could complete his job of leading the humans to victory, which gave them enough time to develop the T-X model and send that back as well. It seems like the humans are losing ground with each film, in fact.
Of course, this doesn't take into account the Fox TV series (which I never got around to seeing, because despite Summer Glau, I just didn't have the time to invest in it) or the fourth film, either of which could contradict this timeline. And of course, there are always other explanations. I still want to do a fifth film in which it turns out that John Connor was never anything more than a red herring, used to divert Skynet's attention while the real leader of the resistance gets on with winning the war. At the end, after defeating Skynet and capturing the time travel facilities, he starts sending back Terminator after Terminator, each one programmed to contact Skynet and give it the vital information that destroying John Connor is the key to victory.
Oh, come on. You know it'd be a great twist.