Skynet was defeated, sent Arnie back in time with the last of it's resources to kill John's mom, the humans managed to get things working just enough to send Reese back, BUT Skynet had ANOTHER secret time travel doohickey, sent a SUPER ROBOT back a little later, so the humans who managed to reprogram an Arnie sent him back TOO....
I wouldn't put Terminator 2 in there. The original sure; it was a no budget flick that came out of nowhere and did an excelent job telling a weird story. (Plus it put Arnie in a role he was MADE for: an expressionless, unemotional robot.) But the second was pretty much just an eyeball massage that actually took away from the original. (So.... Skynet was defeated, sent Arnie back in time with the last of it's resources to kill John's mom, the humans managed to get things working just enough to send Reese back, BUT Skynet had ANOTHER secret time travel doohickey, sent a SUPER ROBOT back a little later, so the humans who managed to reprogram an Arnie sent him back TOO.... How many of these damned time trvel things are there?!?!?!) I'd be tempted to put ALiens in this category too; but it seems like the second one made more of an impact on most people than the first. I would have rated it lower though, since it kinda destroyed sci-fi films. (If I see one more group of space marines creeping through corridors looking for monsters I'm gonna puke!)