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.