Terminator films question?

Ok, after watching all The Terminator films I finally get how it all happened. But one thing still bugs me, through all the films I think even the first one, the saying "There is no fate but what we make", it's even the last scen of Salvation. But if all that is actually supposed to be true, why could John Connor Not stop Judgement day, which was apparently innevitable? Doesn't it seem a bit contradictory to that saying that there was no way to stop Judgement Day at all?