Which is why there was a Terminator targeting Clair Danes.
You send someone into the past, you are either creating a predestinaiton paradox or you vanish to be replaced by a new time line, or you create a new parallel timeline, which effectively means that you can't change the past without killing yourself, or you just plane can't change the past... So why bother?
Skynet sent back three terminators, and the resistance sent back Kyle and T-80.. There wa a TV show? Nooooooooooooooooooooo. Really? There was a TV show?
I liked it when in the TV Show (I'm slowly coming to terms with that this happened) when the resistant fighters started comparing histories only to find that they came form different futures which mean that every time traveller was already having a dramatic impact on the past yet Skynet still always won.