Letter 183 contains one of the strangest (but also most interesting) passages we read for the last class. In it, Tolkien at one point says that he thinks that "even if in desperation 'the West' had bred or hired hordes of orcs and had cruelly ravaged the lands of other Men as allies of Sauron, or merely to prevent them from aiding him, their Cause would have remained indefeasibly right." At first glance, this seems to be an endorsement of Machiavellianism (which would be strange to hear from Tolkien, a devoted Catholic), but if it is it's a Machiavellianism that comes with a number of preconditions and whose actions don't escape judgment at the hands of the Creator. Tolkien seems to be defending these hypothetical actions as oppositions to Sauron, a being he says "represents as near an approach to the wholly evil will as is possible" (Letter 183) within Middle Earth (as it seems he doesn't believe absolute evil is a possibility in the real world, and Middle Earth is our world as far as Tolkien is concerned). This is because ultimately these actions would have come about only because of the threat of Sauron and his legions, and even these atrocities would not have undone the fact that their cause of preventing Sauron from asserting himself as a God-King over Middle Earth was a just one. Tolkien doesn't seem to be making a judgment statement about the men's actions in this hypothetical scenario, but instead about their cause, which he takes to be unsulliable by detestable actions by its champions.
As a result, while initially seeming contradictory to traditional Christian teachings, such as "turn the other cheek," this world view begins to make more sense as a natural consequence of believing in the natural evil of man. If we're willing to accept that "the power of Evil in the world is not finally resistible by incarnate creatures, however 'good'" (Letter 191) and that there is a God who is absolutely good, then it only stands to reason that even if men who follow His will falter and do awful things in His name that God Himself is still absolutely good, and therefore these men's cause (i.e. following the will of God) is just, even if their actions may be reprehensible.
I'll take this as a segue into another topic we discussed in the last class, Frodo's "failure" at Mount Doom. The idea of Frodo succumbing to addiction to the Ring's power or having his will corrupted by that of the will of the Ring was both brought up in class, but I'm less sure now that either of these were Tolkien's intention with that scene. Frodo, like every other being in Middle Earth, is ultimately not wholly good (as he isn't the creator, Illuvatar), is hence flawed, and cannot resist evil. It seems as though Tolkien's intention wasn't that Frodo was overcome by an external evil like the Ring, but "a power of evil which is too great for [the good, even the saintly] to overcome – in themselves." (Letter 192) Frodo wasn't overcome by the Ring or by his addiction, but only by his own natural inclination to evil, an inclination all beings in Middle Earth possess. Frodo was certainly tempted by the Ring's power, but I don't believe that Tolkien's other writings give any indication he thought that Frodo's failure was anything other than a failure of his own will.
This isn't to say that Tolkien thinks that we should look down upon Frodo, as he doesn't see Frodo's failure of will as any different from if his legs had been crushed by a large rock in Mount Doom, leaving him unable to cast the Ring into the lava. As a created being and not the Creator, Frodo's will was never going to be strong enough to allow him to finally cast the Ring into Mount Doom. Their only hope was that once he got to his destination other forces would intervene. Moreover, Tolkien's lack of condemnation for Frodo's failure is very much in line with Christian ideas of salvation and forgiveness. Yes, Frodo was ultimately unable to resist the temptation of the Ring, but this is ultimately a forgivable action, as to do otherwise would be to refuse forgiveness to anyone's sins (as Tolkien seems to be of the opinion that the Ring's power is a temptation too great for anyone with a finite capacity for good to resist once reaching the inside of Mount Doom). Consequently, it seems to me that Frodo's failure in Mount Doom is meant by Tolkien to show us how ultimately we too are doomed to give into the temptation of sin, and it is only by divine providence that we, like Frodo, are saved.