The implausibility of backwards time travel

[PapaPalpatine]

If it ever did become possible to travel back in time and actually affect the timeline, the military and the intelligence community would collectively shit themselves. Particularly, it would open up quite a can of worms where the classifying and declassifying of information is concerned. When information is declassified, it's on the assumption that enough time has passed that it's no longer possible for someone to use said information to cause trouble. You no longer have the luxury of such assumptions once it's possible for someone to hop into a time machine and go back decades or centuries into the past.