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Opening with the WWI Christmas Truce of 1914, where some of the soldiers on the Western Front unilaterally stopped fighting for a few days, the main character, Hal Montgomery, is given a picture by his German counterpart of himself with a British girlfriend along with instructions to give it to the English girl. Hal sees the girl, falls in love his her, but never delivers the picture. The girl, with an illegitimate half-German child, moves in with Hal while still loving the departed German. The novel centers on Hal's deception and his fear of discovery.
Gifts of war highlights the loosening of social barriers as a result of the war. Women are liberated to live in sin, have a good time in bed, take on men's jobs, and cross social barriers. On the down side, the class struggle still exists and a lot of anti-German and anti-British feeling is generated by the horrors of war, foreshadowing WWII.
Other important themes are morality and the balance of love that exists between partners in relationships and marriages.
I found this novel unsatisfying, perhaps because Sam, the British girlfriend, hangs on through most of the novel to the spirit of her German lover -- rubbing in that passion -- in the face of the steady love, care and respect that Hal gives her throughout the war. It's no surprise that Hal doesn't deliver the picture because he thinks that giving it to her will destroy their initially tenuous relationship, and after a while will demonstrate his moral turpitude. It just doesn't sound like a real relationship to me. Then again, I'm not British.