Silhouetted in the foreground is a German submarine, known as a U-boat.  It has just hit a ship, seen behind, on fire.  Germany used submarines with deadly consequences during the First World War.  Maintaining export was vital to the national income and war effort.  Many pre-war trade channels remained open, for instance Welsh tinplate was supplied to France to can military foodstuffs. Pears uses light and dark contrast to great effect in the work.