Hemingway's novel, like many modernist works, challenges the ideal of progress and regeneration that previously characterized American thought. In the aftermath of the war, the traditional pillars of meaning (religion, the state) were destabilized and modernist thinkers sought new moral standards.
Jake, rendered sterile, embodies this new reality. Struggling to accept his condition, he thinks "Try and take it sometime. Try and take it." Hemingway's use of "it" often refers to larger issues. At stake here is not just Jake's wound, but also the wounds of modern times.
The novel opens with an epigraph by Gertrude Stein: "You are all a lost generation." This refers both to those who lost their lives in the war, and also those who lost their moral and spiritual compass. Denied the possibility of romantic love, Jake can no longer rely on the old markers of progress and belief. Similarly, the American Dream, which, as a mythology is strongly dependent on these notions of progress and regeneration, is also sterile. Its empty promise must be replaced by a new philosophy.