Chapter XXI, which takes place at summer's end, is enormously important dramatically, as it introduces two central turning points in rapid succession: Lieutenant Henry being ordered back to the front and Catherine Barkley's revelation that she is three months pregnant.
Structurally, this chapter is one of the most important in the novel. The official letter sent to Lieutenant Henry tells him and us that the story's idyllic central section will soon reach its conclusion. This is especially bad news considering the information related at the start of Chapter XXI: that the Allies are faring poorly not only locally, but also on the Western and Russian Fronts, against the Germans. Worse, the British major with whom Henry chats predicts that the Germans will invade Italy. The scenario that Henry is about to re-enter will be the photo-negative of his joyous summertime love affair with Catherine Barkley.
Obviously Catherine's pregnancy constitutes an enormously significant plot development as well. Now there are consequences to the affair, and responsibilities associated with it. Catherine reasserts her belief, introduced in the prior chapter, that "there's only us two and in the world there's all the rest of them," but this is patently untrue, inasmuch as another life has entered the picture.
In terms of characterization, the notion of Catherine's special bravery is introduced explicitly here. With characteristic modesty, she suggests she would like to be brave, and Henry shows surprising self-awareness by comparing his own bravery to the talents of a mediocre baseball player. (Recall his cocky statement near the start of A Farewell to Arms to the effect that the war could not affect him.) Yet Henry still has living and learning to do: When he naively suggests that "Nothing ever happens to the brave," the more-experienced Catherine counters with the statement, "They die, of course" — ominous foreshadowing, and yet another reason to read on. The novel is like an efficient machine that Hemingway refuels constantly so that it (and therefore our interest in it) remains ever-kinetic.
