World War I is the elephant in the room that nobody wants to mention (yes, it occurred to us that this is probably the only time anyone has ever compared World War I to an elephant). When the war does come up, characters attempt to make flippant comments about it, but there’s a lingering sense of uneasiness – the experience of war is still too fresh in people’s minds to even seriously discuss it. Our protagonist suffered a physical wound that left him impotent as a result of the war; the other characters’ wounds are mental and emotional, and society as a whole is scarred by this global event.
The rupturing event of World War I makes it impossible for Jake to return to America, since he only feels comfortable in a community that shared the traumatic experience of the war first-hand.
In The Sun Also Rises, the central conflict (the impossibility of Jake and Brett’s relationship) is caused by a war-inflicted wound that renders Jake impotent; one might therefore say that the war itself is the main cause of conflict in the novel.