Yeh but if you can travel through time you end up with the paradox of were they created back then or did they travel back to then. If they traveled back in time, from our perspective now they are returning to the solar system, but technically they are not returning. That's the problem with time travel they could of been made in the future traveled back to our time, realised the war was going bad for them and decided to go back further. At that point everything made by them will be ancient ruins in our present time, causing us to believe they are returning back. Sorry if I repeated myself 00:05 my time so a bit tired.