The very first person to reply would have gone back to 1960 to bet on Larsons perfect game- well, please do, I believe he pitched it in 1956......
Some tough Cookies in here -"One bullet for Hitler, one for Stalin"- Might I recommend a slightly different scenario? Give Hitler the Flu bug of 1918 (extremely deadly, it killed more people than the War-given the fact that Hitler was also gassed about that time more than like would have done the job), go talk to Somerset Maugham about his assignment regarding one Vladimir Ilyich Ulyanov (His assignement was to stop said Ulyanov from returning to Russia after the initial Russian revolution of February (March) 1917). He failed to do so, Ulyanov went on to lead the Bolsheiviks in the October 1917 revolution under his Party name, Lenin.
No Hitler, the Nazi remain a no-account minor political party. No Lenin, Kerensky manages to retain power, The Russian Civil war doesn't break out,Stalin goes on to be a zek in Siberia.....