If surrender is signed under duress - say after the bombing of Rotterdam, explicitly aimed at forcing surrender - the legality of occupation is rather questionable. Again, you are arguing submitting to force, not submitting to law.
Where does the resistance strike?
When the Germans landed on Crete, their life was made as miserable as the resistance could make it, in a landscape that was offered lots of opportunity to do so.
If the Jews in Warsaw had not risen, they had stood a good chance of being deported to, then murdered in the death camps - as happened to most of my wife's relatives - they were operating right till the very end.