Certainly, the time Jim spends in prison camps and struggling for mere survival could be moments when Jim discovers the realizations brought on by armed conflict.  Yet, I would that the opening of the book, when the Japanese bomb the European naval forces in Shanghai and intern all the Europeans represent the first moments where Jim discovers "the realisation of war."  One of the major epiphanies that Jim undergoes is the transformation from his understanding of war as something that is a Romanticized vision of nostalgia from one that is forceful in its change and horrific in its consequences.  In an instant, Jim undergoes this transformation as he struggles to survive being separated from his parents and cut adrift in a world where there is little in way of structure and order.  This sense of forlornness and loss might be the first moments in the work, but also might serve as the first instances where Jim discovers "the realisation of war" in terms of what it is and not what Jim thought it to be.