The shoestring paradox. It was always a trime traveller, trying to prevent a war that caused the war that caused the war that they are trying to prevent
It wasn't, but now that somebody came back and screwed up the timeline, the utopian future they originally came from got detached from our causality. Can't get there from here anymore. :(
The question we really need to be asking is why were they trying to alter the timeline in the first place? Presumably they come from a timeline in which the second world war didn't happen so what's their beef with Hitler.
imagine you do that, and the guy still does ww2, and that was your only chance.
Now imagine, you do that, and the guy is born much smarter, and he actually wins, and that was your only chance.
Ofc he still does the war, otherwise why would you go back in time to have sex with some Austrian chick?
(But WW2 was happening anyway, that's why we went back in time to kill that tourist in Serbia)