Sentence:  By contrast , a good deal of nuclear pacifism <WSD>begins</WSD> with the contingencies and the probabilities , and not with the moral nature of the action to be done ; 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:  take the first step or steps in carrying out an action