If the Uruk Army sent by Saruman had captured Frodo and had gotten the One Ring to Saruman, what would Saruman have done with it? Would he have given the Ring back to Sauron or would he have kept the ring and contested Sauron for power over Middle-earth?
Would Saruman have been able to defeat Sauron with the power of the One Ring?
Letter 246 describes a hypothetical confrontation between a hypothetical Ring-bearing Gandalf and Sauron:
Confrontation of Sauron alone, unaided, self to self was not contemplated. One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors. If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever. But the Ring and all its works would have endured. It would have been the master in the end.
Earlier in the same letter we see just exactly how useful the Ring would have been to a hypothetical Ring-bearing Saruman (or anyone else, for that matter):
Sauron would not have feared the Ring! It was his own and under his will. Even from afar he had an effect upon it, to make it work for its return to himself. In his actual presence none but very few of equal stature could have hoped to withhold it from him.
If Saruman had taken and claimed the Ring for his own, Sauron would have been aware of it immediately. We see this happening when Frodo claims the Ring at the Crack of Doom, and there's no reason to think it would be different for Saruman.
What happens next would depend on Sauron himself; whether he would seek to retake the Ring immediately; whether he would visit Saruman himself or send a lesser servant. In any event, a confrontation between Saruman and Sauron seems as though it would not be a closely fought as the one between Gandalf and Sauron described above; Gandalf was able to cast-out Saruman, so it seems as though Sauron would have more easily had the upper-hand in such a contest.
My own feeling is that since it takes time to learn to master the Ring (also Letter 246), Sauron would have had to pay a visit immediately and before Saruman had the time to achieve this mastery. There would have been no need to send an army; this would have been a one-on-one contest in Orthanc. The question of what Saruman would have done therefore becomes moot since it seems inevitable that Sauron would have been the stronger.
It's interesting here that since both Sauron and Saruman were in origin Maiar of Aule, they may be expected to have some knowledge of each other, and that they would have some idea of each other's strengths and weaknesses. That's information that either could potentially use to tip the balance, but perhaps Saruman would also have had awareness that a contest with Sauron was one that he couldn't possibly win? Or perhaps his lust for power would have blinded him to that? We'll probably never know.
I think Saruman would have kept the Ring and used its real and symbolic powers to contest with Sauron for control over Middle-earth by building up his own empire. Sauron would then have suspended his campaign against Minas Tirith, and instead sent his armies north-west, through Rohan towards Isengard. I don't think we've been given enough information to say whether Saruman or Sauron would have won the fight.
In the chapter "the White Rider" (LOTR Book 3, chapter 5), shortly after Ugluk's foray has been defeated, Gandalf gives a detailed analysis of Saruman's motives and Sauron's concerns. He says: "Even reckoned as a lord and captain Saruman has grown very strong... Saruman also had a mind to capture the Ring for himself... [Sauron] now has Isengard to fear as well as Minas Tirith..." The word "fear" seems important here.
After a comment from Gimli, Gandalf adds: "But Isengard cannot fight Mordor, unless Saruman first obtains the Ring. That he will never do now."
Saruman with the Ring would presumably have tried to do what Tolkien says Galadriel or Gandalf might have done, if they have succumbed to the temptation of accepting the Ring:
"they would have built up an empire with great and absolutely subservient generals and armies and engines of war, until they could challenge Sauron and destroy him by force." (Letter 246)
Galadriel and Gandalf each made a conscious decision not to embark on such a course; but Saruman would not have hesitated. Was he not already the warlord of a growing realm, a commander of orcs and men?
How might the Ring have helped Saruman in his empire-building?
Tolkien says in Letter 246 that the Ring is not only comparable to "a devastating weapon" (which Frodo doesn't know how to use) but also to "an object of terror in their religious cult, by which they had been conditioned to treat one who wielded it with servility". For these reasons, Tolkien tells us, the Nazgul (powerful though they were) would have hesitated to attack Frodo directly, after Frodo finally claimed the Ring for his own.
Thus, the power of the Ring is both direct (like the power of a weapon) and symbolic (like the power of a cult object). Both the direct and symbolic aspects could have helped Saruman expand his empire by winning adherents and frightening potential opponents.
I know Letter 246 says Sauron himself did not fear the Ring, would not have hesitated to confront a Ring-wielding Frodo, and could easily have forced Frodo to hand the Ring over. But there are important differences between Saruman's position, if he had taken the Ring in Isengard, and Frodo's position when he claimed the Ring in Mordor.
Frodo was a hobbit with one companion, in the heart of Sauron's realm. Saruman was a wizard and warlord, in a fortress controlled by himself, garrisoned by orcs and men, and separated from Mordor by regions which were not Sauron-friendly.
When Sauron thought Aragorn had the Ring, he reacted with a swift military offensive. In the hypothetic situation where Sauron thought, or knew, that Saruman had the Ring, I think he would have responded in a similar way. Forces from Mordor and Rhun would have been sent to fight their way through Rohan, leaving Minas Tirith undefeated to the south.
Would Saruman have been able to defeat Sauron? I don't think we have enough information to answer this with certainty. We don't know enough about the powers of the Ring, in the hands of an ambitious wizard — how it might have worked as a "devastating weapon" (Letter 246), how, as a symbol, it might in Saruman's hands have affected Sauron's servants and vassals. Nor do we how long would it have taken for Saruman to master the powers of the Ring.
I don't think it would have made Saruman invincible, and, after all, he still would still have had to contend for instance with Treebeard and the Ents, as well as with Sauron. On the other hand, Sauron was not invincible either. If he followed the strategy I've mentioned, his fighters would have to deal with the feisty Rohirrim, and would also run the risk of a flank attack from Gondor, before they could even get near Isengard.
I think the real point has been missed. Even if Saruman won this hypothetical battle, it would be ultimately irrelevant. As was made quite clear at the Council of Elrond, as long as the Ring existed Sauron could not fully be destroyed. So He would do what He did after His defeat by Isildur. His Spirit would continue in some astral realm, and would use His real weapon: TIME. He is Maiar. Like the Valar He was created in the Beginning. No one could even comprehend the length of His existence. So what's a few millennia. He would simply wait and let the Ring do what it was made to do. When given the opportunity It would betray Its wearer, and find Its way back to Its Master.
If Saruman had the ring and he learned to master it, he could rule Sauron's armies and servants as it says in The Fellowship Of The Ring. Also, it says in the book that the Nazgul went to Isengard demanding news of the ring and if Sauraman had the ring, Sauraman said if I had the ring you would bow down to me and call me lord I think Saruman would've won if he captured the ring
