This is clearly a novel that explores the impact of war upon its characters. Hemmingway clearly links war with the loss of innocence in a variety of ways. Consider the various tragedies endured by characters such as Maria, who is raped by Fascist soldiers, or Joaquin who has to grow up very quickly when his parents die. Robert Jordan, too, suffers a loss of innocence, as shown through the contrast between his idealism about the Republican side when he first came to Spain and then his cynicism about the Republican cause later on. Note how Jordan describes his initial feelings of enthusiasm about war:
...you felt that you were taking part in a crusade... [It] would be as difficult and embarrassing to speak about as a religious experience and yet it was authentic... It gave you a part in something that you could believe in wholly and completely and in which you felt an absolute brotherhood with the others who were engaged in it.
This is something far removed from Jordan's cynicism when the reader is introduced to him at the beginning of the novel, showing his loss of innocence through his confrontation with the realities of war.
However, at the same time, Hemmingway makes it clear that it is not only the victims of war that undergo a loss of innocence. Anselmo and Lieutenant Berrendo both have to suppress parts of their humanity through what they have to do to their enemies, killing them and cutting heads of dead bodies. They are changed for the worse as well.
Lastly, Hemmingway deliberately tries to challenge the reader about their own perspective towards war. Just as journalists and writers of war lose their innocence and relinquish their view of war as something involving black and white moral choices, the reader is forced to confront Hemmingway's presentation of war that demonstrates how morality involves no hard and fast "right" and "wrong" positions. This denies the reader of any sense of victory or triumph in the success of the forces of "good" over "bad," as Hemmingway denies the reader any such comforting definitions, as "good" is not a static state but a spectrum that is shown to bleed into "bad," and vice versa.