Sentence:  By contrast , a good deal of nuclear pacifism begins with the contingencies and the probabilities , and not with the moral nature of the action to be <WSD>done</WSD> ; and by deriving legitimate decision backward from whatever may conceivably or possibly or probably result , whether by anyone 's doing or by accident , it finds itself driven to inaction , to non-political action in politics and non-military action in military affairs , and to the not very surprising discovery that there are now no distinctions on which the defense of justice can possibly be based . 
Meaning of the word marked with <WSD> tags:  engage in