Standing up in the trenches wasn't stupid*spoilers*
When Paul stood up in the trenches to draw the bird, he wasn't being stupid. He was caught in the middle of his homes: one where he came from, which he feels further and further from the longer the war progresses, and the one he's in now, in which the war is his new home, but each day he looses more and more of the ones he loves. In a moment of poor judgement and desperation for peace, he stood up to draw the bird. It was his way of escaping both of his worlds for just one moment, which he needed, but unfortunately lead to his death.
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The midget I'm dating could be my daughter! ~Denny Crane