You can push a button and be zapped back in time to meet in private with Hitler or Roosevelt (your choice) in 1940. He doesn't expect you so you must convince him you are genuine. Only your living body (no gags or tricks) can be transported...how do you convince your choice (assume a reasonable person) to take you serious and what information can you give that will decisively change the outcome of the war?
Nothing.
I don't think mucking about with time would be a good idea. If I was close enough to press the button on the time machine, I'd be close enough to destroy it. I'd do my best to completely vaporize said time machine. Then I'd get to work destroying the research that created it.
1. Make sure my living body is as highly trained as possible in unarmed combat and Ninja assassination techniques.
2. Make sure my living body is clad from head to foot in bullet-proof clothing.
3. Kill Hitler. End of problem.
Sadly, I think that Nazi Germany might have been able to fight the Allied forces to at least a stand-still if Hitler had left the war to his generals. There are rumors that he made military decisions by astrology. At best, he was very incompotent as a military strategist, regardless of where his "insight" came from. I'd be more inclined to kill some of the more skilled Axis generals or scientists. But, if Germany had folded like an accordian because the best Nazi Generals were killed, would Stalin have been able to grab even more territory than he did after WWII? Maybe all of Europe?
If Roosevelt had been warned in 1940 and wiped the floor with the Japanese fleet that attacked Pearl Harbor, would America have been motivated and mobilized to the extent it was due to the damage inflicted at Pearl Harbor? With no single event to rally popular opinion, would America have supported getting involved in Europe whatsoever? Maybe. Doubtfully.
1940 would have been too late to convince Roosevelt not to pass the National Firearms Act, also. Drat.
