The first times I read the Lord of the Rings I absolutely loved it of course, but something in it nagged at me. Why was the Sammath Naur unguarded? How costly would it have been for Sauron to staff it with, say, 5 orcs? Sam and Frodo’s incredibly arduous journey, not to mention the efforts of the rest of the Fellowship and mankind in general, could have all amounted to naught if Sauron had just defended the entrance to the one place in Middle-Earth where the Ring could be destroyed. I reasoned that it was protected: the Morgai was a daunting barrier, inside of which Mordor teemed with orcs. However, it seemed reasonable for Sauron to place at least a couple of orcs guarding the entrance to the mountain! He has orcs running around all over Arda, but can’t spare some at Orodruin? I would have been kind of disappointed if after 900 pages the quest had failed, but still, it lacked some realism. The answer lies in the nature of evil in Tolkien’s mythology, specifically the stories and motivations of Morgoth and Sauron.
Morgoth and Sauron at their core try to supplant Ilúvatar. Morgoth begins residing in the shadow (the Void) implicitly created by Ilúvatar’s creation of good from the beginning, creating discordant themes in the Ainulindalë:
“He had gone often alone into the void places seeking the Imperishable Flame; for desire grew hot within him to bring into Being things of his own, and it seemed to him that Ilúvatar took no thought for the Void, and he was impatient of its emptiness. . . But being alone he had begun to conceive thoughts of his own unlike those of his brethren.
Some of these thoughts he now wove into his music, and straightway discord arose about him. . .”
He greatly desired to do and create independently of Ilúvatar, and so sought independence by means of the Void. Ultimately, of course, Ilúvatar said that “no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite.” In addition, “he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.” No matter what evil Morgoth or Sauron attempt to perpetrate, in the end they are still participating in the ultimate Good that is Ilúvatar’s creation (a neat example being Sauron’s orders to capture halflings leading directly to the speedy deliverance of Merry and Pippin to the Ents, who were instrumental in the Saruman’s downfall). Morgoth sought power and domination over Arda: he wanted to be God, Creator instead of merely sub-creator, and therein lay his Evil.
Sauron’s obsession lay less in God status and more in the domination of wills. He was worshipful of Morgoth and Sauron recruited others to worship him as well. His gift of knowledge to the Elves was a duplicitous act, enticing them with rings of power in order to bind their wills to his by his creation of the One Ring. As is seen in Frodo’s long journey with the One Ring, it attempts to bend the will of the ring bearer and make him succumb to Sauron. It is constantly pressing Frodo to wear it, and thus reveal himself to Sauron, and only through great acts of will is Frodo able to resist. Even this will can’t stop him in the end though, and at the Cracks of Doom Frodo refuses to give it up, being saved in the end by his own acts of pity towards Gollum, who bites the Ring off of his finger and falls into the fires of Doom while gloating. Men fell to similar temptations, attempting to reverse their mortal fates. Rather than seeing their fate of mortality as a gift in the same way that immortality was a gift to the Elves by Ilúvatar, they longed for immortality. Sauron was able to use this against them, giving them 9 rings which gave them a form of immortality, but also bound them permanently to his will.
So how does this help me with Sauron’s overlooking of the entrance at the Cracks of Doom? It never occurred to him that the for Good in the world would not attempt to use the Ring. Or maybe it did, but the thought of them not attempting to use its enormous power was inconceivable. He was following the model of those before the end of the Third Age: the kings of Men who lusted for power and immortality, Isildur who wavered at the last possible moment, Gollum etc. The King returned, but he was not as corruptible as the Kings of old that lusted for power and immortality. The forces that showed up to defend Arda from Sauron were full of incredibly humble people: Aragorn, Sam, Gandalf, and others who were faced with the temptation of the Ring were able to resist. Gandalf’s position was especially elucidating. He reasoned that he would attempt to use the Ring for good and not evil, but the power of dominating wills such as the Ring gives to the bearer is inherently an evil one. Had he forced people into Good, it would have made it seem Evil. People need to be able to choose Good, otherwise it ceases to be Good. Evil will always exist in the shadow of Good. They of the Council were wise enough to see that the Ring had to be destroyed. They realized that power gained through the domination of free will would be a dark one, and so they sought a more humble victory. Sauron thought they would want a tyrannically good Arda, not an Arda left up to its own machinations, which in the end is what happened, just how Ilúvatar would have wanted it.