In chapter eight of A Separate Peace, by John Knowles, Phineas (Finny) is back at Devon after having been home, recuperating from the shattered leg he got when Gene "jounced" the limb of the tree they were both on. Finny is back, but things are different; Finny is different.
Finny is on crutches and can no longer do the things he once did; he is also at least a little distrustful of Gene, though he does not want to believe that his friend, someone he loves and assumes loves him the same way in return, would have deliberately hurt him. Finny also used to take the war quite seriously.
But now Finny has changed his view, perhaps because he will never be able to participate in the war and is resentful about that. When Gene talks about the war as if it were real (which of course it is), Finny asks incredulously, "Have you really swallowed all that war stuff?" Gene knows the war is real; however, Finny claims that a bunch of "fat old men" sat in a room somewhere and invented the war as an attempt to keep the young people of the world busy and out of trouble.
This is such an abrupt change in Finny's thinking that Gene teasingly asks him how it is that the entire world believes in the war but only Finny knows the truth. Finny's answer startles them both: "Because I've suffered." They are both surprised at this stark admission of bitterness which Finny never used to feel but feels so intensely now.