I am "warring" with another country in Primary. But after I attack him a couple of times his NW drops below and I get 'Humanitarians will not allow you to attack such a small opponent!' Then after I sell my military on the market, I drop in NW low enough for him to attack me with all his turns. But the next day I can only attack him a couple of times......

You will have to reschedule you log in time so that you log in when your goods are sold. When your good sell you drop in NW attacked him and then grow out of his range so that he cannot hit you when he logs in.

And then you repeat tomorrow and the next.

Pm me, andy, ill give you some pointers ;)