"For Esmé" plays upon a common theme of war fiction, the idea of the American soldier abroad. The way people elsewhere (in this case, in England) view Americans is a central theme here. When looking back on World War II, Americans tend to view themselves pretty unilaterally as the good guys, but this story, written by an American author, highlights the fact that, even though US soldiers were fighting on the right side, it doesn't mean that they're all heroes. In fact, the story examines the whole idea of wartime heroics in a questioning light.
The tension between the American soldiers and the English civilians they interact with further destabilizes the myth of wartime heroism.
The narrator's unconventional sympathy for the arrested German woman whose house he and the other soldiers live in demonstrates his understanding of the suffering inflicted on people on both sides of the war.