WWII was a long and bloody conflict, stretching from 1935, 1937 or 1939 depending on where you start it to final victory in 1945. It killed around 73 million men, women and children many of whom never picked up a gun. It was a horrible conflict, but lets extend it. Have WWII last as long as possible. It can start whenever and end whenever but it has to be a distinct conflict from WWI; so no "WWI just never really ends" or something. Also try and keep the fighting as contiguous as possible with few if any times of informal truce between the powers. Lets not have it not end simply because Germany and Lichtenstein simply forgot to sign a treaty or some technicality like that to.
Bonus: No nukes and no total extinction of humanity.