Frederic's an idealist. As a quintessential Hemingway hero, he wants to prove himself as a man, and he figures that the best place to do this is on the field of battle. Participating in World War I will give him a chance to find a place for himself in life, a common desire of many young men of that particular period. But as the United States wasn't involved in the war at the time, he chose to join the Italians, who were part of the Allied war effort.
Frederic isn't just doing this for himself, however. He genuinely identifies with the Italian people, so much so that he feels like he's almost one of them. Certainly that's how some of the Italian soldiers feel about him. This is what happens when one's personal ideals are not matched by the actions of one's government. In such a situation, it often becomes necessary to identify with another nation's struggle and to become completely immersed in their national culture and destiny. That's what Frederic has done.
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