I've been thinking recently it would be helpful if we could leave the war room the same even when all generals are out. Right now I attack and send 4 generals, then the window where you select troops goes away. It just says you have no generals.
My suggestion is to leave this window up (I'll call it the calculator), so we could still enter values for all fields, even if there are no available generals. If you try to attack, you would obviously get a similar error message as if you try to send army you don't have. It could even still have the same prompt that says you have no available generals, but still keep the calculator active since it is great for attack calculation.