I've long heard a secret protocol already existed between Mexico and Germany dating back to WWI where Mexicans would grab the southwest as a distraction in the event of a German invasion of North America.
Second, the Third Reich was not a democracy; Hitler was not going to be voted out of office because of some battlefield disaster. The German people were too cowed to express disapproval of Der Fuhrer (consider Stalingrad).
Guess you must have missed the part where Hitler’s own people tried to take him out at least once.
Not in the least bit secret: http://en.wikipedia.org/wiki/Zimmermann_Telegram