So many people that confronted with the postulate “If you could go back in time what would you do?” would say “Kill Hitler”. Granted it’s not the worst answer, but I would go much earlier than that. Specifically to Baghdad year 1258 when the city was conquered and ransacked by Mongols under the command of Hulagu Khan (grandson of Genghis Khan). When they conquered Baghdad the Mongols utterly destroyed hospitals, homes and the city’s prolific library called Bayt al-Hikma (House of Wisdom) which was a translation institute and research center and held copies of all western and Arabic literature on all subjects. Witnesses to the Mongols’ destruction claim the rivers of Tigris ran black with the ink of the destroyed manuscripts under half a year. Who knows what great books we’ve lost?
When I get a time machine I will take a 3D projector (battery operated of course by Duracell, one cannot trust those ancient Persian batteries) and a huge canvas and put it outside the western walls of Baghdad and display Avatar on it. I imagine the Mongols will flee in panic because of this mighty sorcery and just ransack Russia … or whatever, not like the Russians didn’t have it coming anyway.