In the opening scene while walking through the city the soldiers came across lot of pamphlets/fliers. It had a picture depicting the perimeter which Germans had formed in the sea to surround/trap the entire English/French armies. It had written on it - "We surround you".
Were such fliers/pamphlets really distributed by Germans across the city? Or it was just a way of exposition? (Nolan has done such things in his past movies. Remember the wormhole explanation from the movie Interstellar?)