The war is symbolic of the end of innoncence Although the boys spend much of their time at Devon preparing for the war, their lives remain rather sheltered and detached from the reality that they will actually have to fight. Finny even goes as far as to say the war isn't even real. This shows he is rebelling against the idea of growing up and facing the real world. When characters, such as Leper, get sent off to war, we see a sudden end of innocence, as they get exposed to the atrocities that war presents.
The war symbolizes the future.