I think disappointed here means suppressed according to the context. Many times, hopes for a speedy end to the war, which had been fanned by optimistic rumors, were suppressed . Hopes were suppressed at the time; therefore some men lost all hope...
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Stenka25Many times, hopes for a speedy end to the war, which had been fanned by optimistic rumors, were disappointed.The subject of that sentence is HOPES, specifically HOPES for a speedy end to the war. The prisoners are hoping that the war will end soon, and they hear rumours which RAISE their hopes, i.e. make them think the war is more likely to end soon.