Why isn't war discussed in the last book, and is it still about war?
In Book 5 Frederick Henry and Catherine Barkley leave Italy and the war behind as they escape into Switzerland. The focus now is on their time together as they await the birth of their child. Rather than showing the harsh realities of war, Hemingway provides an intimate view of the couple's isolation as they are virtually snowbound in the mountains. The tone changes, however, as Catherine's time nears and they must move from the mountain down to the village, characterized by mud, ominously foreshadowing the doom to come.
The final book is indirectly about war because the novel's title fulfills its double meaning here. The deaths that Henry has seen during his war experiences do not prepare him for the deaths of his son and beloved Catherine. He is able to escape the war into a neutral territory, but he cannot escape death. He must say farewell to the arms of the woman he loves.
Hemingway's A Farewell to Arms is about war, but it is also about a man (Frederic Henry) who re-evaluates what he is doing in a war that he really doesn't support—these are not his people. When he becomes injured and consequently falls in love with his nurse (Catherine Barkley), his entire outlook on the war and life change to the point that he is willing to desert the army and "run away" with her to Switzerland.
The story does take place during a war and Frederic must decide how he feels about war and Catherine, but more so, it may reflect a man's search to find his place in a world that happens to include war, love, and ultimately, loss.
The war in book 5 becomes an internal conflict as well as an external one. Henry believes he is escaping the war--making a conscious choice of saying "farewell" to the military arms of guns and ammo. However, he ends up saying an involuntary "farewell" to the arms of his love and their child in the end. So, war images are definitely present, but this is the war inside his head caused by the loss of his family.