This fact had always bugged me. When Frodo finally goes to the Cracks of Doom he is met with practically no resistance. This seems uncharacteristic of Sauron which in my humble opinion makes it a tiny flaw in the plot. Just to be safe, Sauron could have heavily guarded Mount Doom to prevent the destruction of the Ring: why didn't he do so?
It is quite simple: Sauron did not expect, and could not conceive, anyone would actually try to destroy the Ring instead of claiming it for themselves.
"He is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place.That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream."
-The Two Towers, "The White Rider"
This complete lack of desire of any kind to master the Ring was the primary reason Frodo was chosen to be the Ringbearer. Even Gandalf refused to touch the Ring for fear of being consumed by it and not being able to destroy it.
Edit: During a re-read of The Fellowship of the Ring I have come across these other relevant passages:
Well, let folly be our cloak, a veil before the eyes of the Enemy! For he is very wise, and weighs all things to a nicety in the scales of his malice. But the only measure that he knows is desire, desire for power; and so he judges all hearts. Into his heart the thought will not enter that any will refuse it, that having the Ring we may seek to destroy it. If we seek this, we shall put him out of reckoning.
And this account from Isildur who could not even bring himself to put the ring in a fire:
The Ring misseth, maybe, the heat of Sauron's hand, which was black and yet burned like fire, and so Gil-galad was destroyed; and maybe were the gold made hot again, the writing would be refreshed. But for my part I will risk no hurt to this thing: of all the works of Sauron the only fair. It is precious to me, though I buy it with great pain.
Both excerpts from The Fellowship of the Ring: Book II: Chapter 2: The Council of Elrond.
This was a key point of Gandalf and Aragorn's strategy, and the whole reason they led the army of Minas Tirith to the Black Gate of Mordor. The hope was to draw not only Sauron's attention, but his armies as well, leaving Mordor itself unguarded.
Book 5, ch. 9:
[Gandalf]: ‘His doubt will be growing, even as we speak here. His Eye is now straining towards us, blind almost to all else that is moving. So we must keep it. Therein lies all our hope. This, then, is my counsel. We have not the Ring. In wisdom or great folly it has been sent away to be destroyed, lest it destroy us. Without it we cannot by force defeat his force. But we must at all costs keep his Eye from his true peril. We cannot achieve victory by arms, but by arms we can give the Ring-bearer his only chance, frail though it be.
In addition to all the other comments, guarding Mount Doom is difficult. It regularly erupts, so having soldiers on guard duty near to it presumably raises the very real risk of them being burnt to a cinder. The road to Mount Doom had to be regularly cleared and maintained at great effort according to the books, so the most likely group of Saurons servants you were likely to find there would probably be a highway repair crew...
Mordor itself was actually heavily guarded, with legions of Orc troops occupying it, as we learn in The Land of Shadow:
Frodo and Sam gazed out in mingled loathing and wonder on this hateful land. Between them and the smoking mountain, and about it north and south, all seemed ruinous and dead, a desert burned and choked. They wondered how the Lord of this realm maintained and fed his slaves and his armies. Yet armies he had. As far as their eyes could reach, along the skirts of the Morgai and away southward, there were camps, some of tents, some ordered like small towns.
The fact is that Frodo and Sam had taken a little-known back-door into Mordor; the main entrance (via the Black Gate) was effectively impassable (from The Black Gate is Closed):
Across the mouth of the pass, from cliff to cliff, the Dark Lord had built a rampart of stone. In it there was a single gate of iron, and upon its battlement sentinels paced unceasingly. Beneath the hills on either side the rock was bored into a hundred caves and maggot-holes: there a host of orcs lurked, ready at a signal to issue forth like black ants going to war. None could pass the Teeth of Mordor and not feel their bite, unless they were summoned by Sauron, or knew the secret passwords that would open the Morannon, the black gate of his land.
And even the back-entrance they took was also well-guarded (The Tower of Cirith Ungol):
Since his return to Mordor, Sauron had found it useful; for he had few servants but many slaves of fear, and still its chief purpose as of old was to prevent escape from Mordor. Though if an enemy were so rash as to try to enter that land secretly, then it was also a last unsleeping guard against any that might pass the vigilance of Morgul and of Shelob.
In order to actually do anything useful at Mount Doom, you obviously have to get into Mordor first, which is something that one does not simply do.
Without Gollum's intervention Sauron's reaction time would have been fast enough to have prevented the ring from being destroyed.
In a 1963 letter written in response to a reader's inquiry about Frodo's failure to surrender the Ring in the Cracks of Doom, Tolkien discussed what would have happened had Gollum not been there to instantly steal and destroy the ring after Frodo claimed it.
He explains that Sauron's reaction when Frodo claimed the Ring was to sent the Nazgul to Mount Doom, where they would feign allegiance to Frodo as the new owner of the Ring, with the purpose of getting him outside of the Crack so they could destroy the entrance and/or stalling for time until Sauron could get there and defeat Frodo 1v1.
When Sauron was aware of the seizure of the Ring his one hope was in its power: that the claimant would be unable to relinquish it until Sauron had time to deal with him. Frodo too would then probably, if not attacked, have had to take the same way: cast himself with the Ring into the abyss. If not he would of course have completely failed. It is an interesting problem: how Sauron would have acted or the claimant have resisted. Sauron sent at once the Ringwraiths. They were naturally fully instructed, and in no way deceived as to the real lordship of the Ring. The wearer would not be invisible to them, but the reverse; and the more vulnerable to their weapons. But the situation was now different to that under Weathertop, where Frodo acted merely in fear and wished only to use (in vain) the Ring's subsidiary power of conferring invisibility. He had grown since then. Would they have been immune from its power if he claimed it as an instrument of command and domination?
Not wholly. I do not think they could have attacked him with violence, nor laid hold upon him or taken him captive; they would have obeyed or feigned to obey any minor commands of his that did not interfere with their errand – laid upon them by Sauron, who still through their nine rings (which he held) had primary control of their wills. That errand was to remove Frodo from the Crack. Once he lost the power or opportunity to destroy the Ring, the end could not be in doubt – saving help from outside, which was hardly even remotely possible. Frodo had become a considerable person, but of a special kind: in spiritual enlargement rather than in increase of physical or mental power; his will was much stronger than it had been, but so far it had been exercised in resisting not using the Ring and with the object of destroying it. He needed time, much time, before he could control the Ring or (which in such a case is the same) before it could control him; before his will and arrogance could grow to a stature in which he could dominate other major hostile wills. Even so for a long time his acts and commands would still have to seem 'good' to him, to be for the benefit of others beside himself.
The situation as between Frodo with the Ring and the Eight (The Witch-king had been reduced to impotence.) might be compared to that of a small brave man armed with a devastating weapon, faced by eight savage warriors of great strength and agility armed with poisoned blades. The man's weakness was that he did not know how to use his weapon yet; and he was by temperament and training averse to violence. Their weakness that the man's weapon was a thing that filled them with fear as an object of terror in their religious cult, by which they had been conditioned to treat one who wielded it with servility. I think they would have shown 'servility'. They would have greeted Frodo as 'Lord'. With fair speeches they would have induced him to leave the Sammath Naur – for instance 'to look upon his new kingdom, and behold afar with his new sight the abode of power that he must now claim and turn to his own purposes'. Once outside the chamber while he was gazing some of them would have destroyed the entrance. Frodo would by then probably have been already too enmeshed in great plans of reformed rule – like but far greater and wider than the vision that tempted Sam (III 177) – to heed this. But if he still preserved some sanity and partly understood the significance of it, so that he refused now to go with them to Barad-dûr, they would simply have waited. Until Sauron himself came. In any case a confrontation of Frodo and Sauron would soon have taken place, if the Ring was intact. Its result was inevitable. Frodo would have been utterly overthrown: crushed to dust, or preserved in torment as a gibbering slave.
September 1963 Letter to Mrs Eileen Elgar
In the book itself we see a brief hint of this, as we see the Nazgul leaving the battlefield as soon as the ring gets claimed.
From all his policies and webs of fear and treachery, from all his stratagems and wars his mind shook free; and throughout his realm a tremor ran, his slaves quailed, and his armies halted, and his captains suddenly steerless, bereft of will, wavered and despaired. For they were forgotten. The whole mind and purpose of the Power that wielded them was now bent with overwhelming force upon the Mountain. At his summons, wheeling with a rending cry, in a last desperate race there flew, faster than the winds, the Nazgûl the Ringwraiths, and with a storm of wings they hurtled southwards to Mount Doom.
The Lord of the Rings - Book VI Chapter 3 - "Mount Doom"
But the Nazgûl turned and fled, and vanished into Mordor’s shadows, hearing a sudden terrible call out of the Dark Tower; and even at that moment all the hosts of Mordor trembled, doubt clutched their hearts, their laughter failed, their hands shook and their limbs were loosed. The Power that drove them on and filled them with hate and fury was wavering, its will was removed from them; and now looking in the eyes of their enemies they saw a deadly light and were afraid.
The Lord of the Rings - Book VI Chapter 4 - "The Field of Cormallen"
A thief steals your 10 million dollars from you. You are busy searching for him. He comes back and burns it at your home’s Chimney. Why didn’t you place guards to prevent that is beyond me.
You got my point.
Destroying the ring was easy to think about for us readers. But that alone was a stroke of genius from Gandalf/Tolkien nobody will ever fully grasp.