Sentence:  Captain Edward Carlisle...felt a curious sensation of helplessness seize upon him as he met her steady gaze,nb...; he could not tell what this prisoner might do. He cursed the fate which had assigned such a <WSD>duty</WSD>, cursed especially that fate which forced a gallant soldier to meet so superb a woman as this under handicap so hard. 
Meaning of the word marked with <WSD> tags:  That which one is morally or legally obligated to do.