One of the first things you notice in the story is the loneliness of the characters. This loneliness is reinforced by the setting of the story. First, it is told by an American who is isolated in an Italian hospital. The hospital itself is located "across a bridge" from the town. When the officers go to the town, they are not well-liked. This separates the main character even further. Finally, notice the title of the story "In Another Country". The narrator is in a strange country, and has even been taken away from the war he came to fight. All of this leads to a mood of loneliness and sadness.
Hemingway's story of physical injury and emotional loss is a somber short story that successfully conveys a mood characterized by grief and suppressed sentimentality and a sense of subterranean angry refusal to accept loss. As a result of this dual sensibility, the mood of "In Another Country" might be described as one of fragile calm.
In keeping with the norms of describing mood with emotional and psychological terms, we might apply depressed, detached or remote to characterize the mood of this Hemingway story. Maybe the best term to use here, however, is afflicted.
While the turmoil experienced by the characters is internal, their injuries come from outside of themselves. They are affected by a brutal world and thus suffer the afflictions of that brutality and of the indifference of events.
The robotic methods of physical therapy that are featured in the story parallel the purposeful indifference that these two characters cultivate in regards to their injuries. They are literally going through the motions of recovery by the end, seemingly as far removed from any hope for true recovery as they are from the hope for glory in battle.
There was a time when none of us believed in the machines, and one day the major said it was all nonsense.
The protagonist, like the major, is experiencing rather severe alienation. The protagonist is far from home, having been injured during war in a foreign country. The major is from Italy, and so is at home in the country, but in addition to having lost some of his physical abilities to injury, the major also loses his wife.
"He cannot marry. He cannot marry," he said angrily. "If he is to lose everything, he should not place himself in a position to lose that. He should not place himself in a position to lose. He should find things he cannot lose."
He spoke very angrily and bitterly, and looked straight ahead while he talked.
The mood of the story is charged, ultimately, with a tension that animates the sense of alienation and loss and poses that isolation as a sort of existential suffering. In order to live fully and with joy, one must take the chance that one's joy can be taken away.
As much as the narrative may strive to suppress the emotional nature of this insight, it also presents the insight as the central motif of the story. Hemingway gives us a picture of two kinds of romance (one of war and one of love) and suggests that despite the some nobility in the pursuit of romantic notions, deep suffering is unpleasant and, if still noble, bereft of the ephemeral hopefulness that initiated the pursuit.