Sargent William Furia, of Philadelphia, PA, during the Battle of the Bulge.
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Quote from the February 9, 1945 edition of the Windsor Daily Star:
"Sargent William Furia...wears a piece of a fancy lace curtain for a helmet cover. Starting as a joke, the lace decoration has proven practical as snow-camouflage cover in Luxembourg.”
He's smiling because, by this date, the Germans had been beaten back, and the Battle was essentially over. And, because he's alive.
Source: retronaut.co