Most usually in fiction an antagonist is a human character who opposes or works against the protagonist; however, sometimes an antagonist can be an entity, such as an organization or some type of force. In this novel, Lt. Frederic Henry's antagonist is World War I as it is being fought on the Italian front. It is the war that most threatens him and succeeds in almost killing him.
After being wounded, Frederic recovers and returns to the war. He functions with grace and courage, discharging his duties as an officer in a conscientious and professional manner. During the disastrous retreat from Caporetto, however, Frederic finds himself in a line of soldiers crossing a bridge over the Tagliamento River. Ahead of him he sees Italian officers being questioned by the carabinieri who are looking for deserters and infiltrating enemy troops. Frederic witnesses officers being executed. When he is pulled out of line because he speaks Italian with an accent, Frederic knows he is about to die. He dives into the river to escape execution.
Making his way through Italy hidden in a railroad flat-car, Frederic at this point does see himself in a profound new way. He had run to avoid being shot, but lying on the floor of the car under a canvas cover, he makes a conscious decision to desert. He is through with the war:
Anger was washed away in the river along with any obligation. Although that ceased when the carabiniere put his hands on my collar. I would like to have had the uniform off although I did not care much about the outward forms. I had taken off the stars, but that was for convenience. It was no point of honor. I was not against them. I was through . . . it was not my show anymore and I wished this bloody train would get to Mestre and I would eat and stop thinking. I would have to stop.
Frederic no longer sees himself as an officer with responsibilities to his men. Through no fault of his own, he has been separated from his men and has lost his ambulances. He claims a new identity, his own. His uniform is now foreign to him. He has cut off the stars to avoid apprehension as the deserter he has chosen to be; later he would notice that the cloth bears the outline of where they had been, symbolizing the life he had left behind.
This new identity does not come easily to Frederic as he considers it. It marks a profound difference in him. He knows he will have to stop thinking about the choice he has made.