How is Americanism shown in this story?

One could argue that the unnamed lieutenant's extraordinary bravery is an example of Americanism. Americans have traditionally expected their men to be brave, especially on the field of battle. And there can be no doubting the lieutenant's bravery here. Despite being seriously wounded, he doesn't make a fuss over his predicament; he simply seeks medical attention without uttering so much as a murmur of complaint to anyone.
Although the lieutenant expresses fear at the prospect of his arm being amputated, he's not so much frightened of the pain that he'll be forced to endure but rather the attack on his bodily integrity that losing a limb will entail. Like any good American, he deeply cherishes personal freedom and instinctively recoils at anything that might impede or compromise that freedom.