How many would be lost because the U.S. would then not enter WWII, including Chinese, Filipino, British, French, Korean, Jewish, and Russian?
Nah, war is always awful, but how do you stop hellbent madmen or power hungry dictators with big, well equipped, technologically advanced, and motivated militaries? Really, the only way would be to throw another such thing back at them, hopefully a little bigger, more equipped, advanced, motivated, and from a free nation.
Besides, I hate time travel stories... and as much as I like it, I sometimes am uncomfortable even with "City on the Edge of Forever" (yeah, I know, sacrilege).