Tolkien says in the introduction: 
	Quote:
	
	
		| This act of pride and misplaced chivalry proved fatal. | 
	
    That refers to the fact that he allowed the enemies to cross the ford to give them a fair chance, which proved to be his downfall.  Fairness in war is folly, is the moral of the story, I guess.
		
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				'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth..
.'