Hmm.....good question. I believe I would do the following: I appear in his office following a major loss early in the war in a German uniform, after the social convention of explaining I am from the future and proving so (trust me I have methods to do so) in German, he will ask why I am there. That is when I tell him I am there to inform him of important news from the future, and then I come in real close and say (in German, so he understands): YOU LOST! Then, stripping off the German uniform to reveal a red, white and blue suit, I punch him into a cupboard (where I smuggled in speakers) where the Star-Spangled Banner (or God Save the Queen depending on whether or not the U.S had entered the war) plays. I lock the door for some officer to find him later, and walk off into the sunset, proud of what I did, hoping I did enough to shock him into a downward spiral, ending the war sooner. That is all.