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In order to buy time for Frodo to reach Mt. Doom, and to distract Sauron from the peril in his own land, Gandalf and Aragorn led a small army to the Black Gate, making Sauron believe that Aragorn did indeed intend to challenge him directly. Reacting swiftly, Sauron sent all his armies to the Black Gate, to utterly crush the Men of the West, and regain his prize.