One of the themes of A Farewell to Arms is the inexorable path of fate. It is fate that first throws Catherine, a British nurse, and Frederic, an American serving in the Italian army, together. When he is wounded, he spends time in the hospital where she is working, and they get to know each other and fall in love.

Their relationship is continually interrupted by the war, another way in which fate constantly intervenes in their lives. After Frederic recovers, he is sent back to the front, where he experiences a disastrous Italian campaign and is pursued by the authorities for his role in the campaign. He and Catherine must escape to Switzerland.

There, she gives birth to a stillborn child and dies soon afterward. It is fate that causes them to need to flee and that causes her death as well as that of their child. Fate takes many forms in this book; Catherine and Frederic's love affair is affected by the war and, in the end, by her health. Catherine and Frederic cannot escape the inexorable path of fate.

Hemingway's Farewell To Arms deals frequently with the topic of war, masculinity, death, existentialism, and love. The novel's main theme, however, seems to be mankind's inescapable placement against two polar opposites, love and violence.

Henry experiences horror, utter brutality, and gruesome violence throughout his time served in the war. He and his fellow soldiers are often broken down and shaken to their core by the senseless violence that surrounds them. They aim to be traditionally masculine men, brave and heroic, but more often than not they buckle under the pressure.

Juxtaposed against these scenes are passages depicting Henry's limitless love for Catherine. Where war offers acts of violence, torture, and murder, their love offers acts of sexuality that, while physical, couldn't be further from the savagery of war. Furthermore, much of the couple's love comes from the violence; Catherine's fiancé is killed, and she buries her sorrow in Henry. Meanwhile, Henry, unable to cope with the depressing nature of his experiences, escapes from the horrors through his love for Catherine. They aim to escape the war together, literally and figuratively. They continuously find themselves deeply in love, scarred by the war, and unable to escape tragedy.

When I first read this story, I considered the horrors of war to be the primary theme.  However, there is an excellent link given below to an eNotes page that details all of the themes of "A Farewell to Arms" - identity, individualism, patriotism, and war.  Give that a close read, then give the story a re-read to more fully understand all of the themes.  Good luck!

What is the main theme in A Farewell to Arms?

Numerous themes are developed in the novel, each contributing to Hemingway's examination of man's essential condition, one of profound spiritual loneliness. Through Frederic, and to a lesser degree through the other characters, Hemingway develops the idea that life is nothing more than random, often tragic, events, occurring without any divine plan. Various characters in the novel often speak of God, but He is nowhere to be found; "religion" becomes an expression of personal need and is defined, pursued, or practiced differently by the characters. Absent God, Frederic and Catherine embrace each other. "You are my religion," Catherine tells Frederic.

The spiritual loneliness that pervades the novel is seen in the characters' attempts to cope with life, to either find meaning in it or to escape it. Rinaldi operates, drinks, and sleeps with many women; the priest prays; Count Greffi longs to become "religious" before he dies; Catherine dedicates herself to Frederic; and Frederic faithfully performs his duties as an officer, until he loses his ambulances and his men and is about to be shot during the insanity of war. Hiding under a tarp on a railroad flat car, Frederic declares a "separate peace" from the war and makes his way back to Catherine, who has become his anchor in a chaotic world without meaning. With her death, however, he is left alone. The brutality and futility of life is summarized in the novel's conclusion as Frederic waits for Catherine to die:

Now Catherine would die. that was what you did. You died. You did not know what it was about. You never had time to learn. They threw you in and told you the rules and the first time they caught you off base they killed you. Or they killed you gratuitously like Aymo. Or gave you the syphilis like Rinaldi. But they killed you in the end. You could count on that. Stay around and they would kill you.

Following this meditation, Frederic remembers watching ants die in a fire. He could have been their "messiah" and saved them, yet he had watched their suffering objectively and acted only to further their destruction. After praying in desperation to a god that does not save Catherine, Frederic's spiritual loneliness is complete. He walks back to the hotel in the rain, alone in the world with nothing to sustain him except his own courage. There is no salvation; life is a condition only to be endured.