Tony Williams' book The Future War has generated some commentary here and elsewhere on what could have occured from late 1935 to the original beginning of WW II. If you were in that position with one nation (and, knowing that at least some, if not many, other nations had similar knowledge available) what would you recommend they change in their original strategies and technologies?
Now, remember you are still dealing with the original political and military leadership of your chosen nation and should work within those limits.
I'll post my version in a few days after everyone gets a shot at this first. Let's see how inventive we really are....