Tolkien was clear in the stories of The Silmarillion and The Lord of the Rings that the struggle between good and evil is never-ending. No sooner did the Valar vanquish Melkor than Sauron emerged in Middle-earth and forged the Rings of Power to bring it all under his control. If history in the first Three Ages are any indication, one can assume that no sooner do the heroes of The Lord of the Rings vanquish Sauron at the end of the Third Age than a new, perhaps worse promoter of evil will arise in the Fourth Age (though Tolkien gives no indication of this in his brief chronology of the Fourth Age).
Not only do the seeds of evil continue to sprout and grow in Middle-earth, but the dark conditions in which they flourish continue to spread. As the sources of light diminish over the different Ages of Middle-earth, it becomes easier to deny the power of light and tout that of darkness. This expansion has fostered a loss of hope and a lack of faith that helped defeat the various Elven kingdoms in Beleriand, and later Númenor. After that, it sapped the strength from the exiled kingdoms of Arnor and Gondor.
In our world, a feeling of growing darkness can give rise to ennui and defeatism. Inherent in this defeatism is the sense that evil is stronger and somehow more real than goodness. This is, of course, the precise opposite of the truth as many, including Tolkien, see it: Evil cannot prevail over goodness because evil is just a denial of the only true and fundamental wisdom: God, in the Judeo-Christian and Islamic vocabulary; Enlightenment in the Buddhist.
In other words, the light may be hidden or blocked, thereby allowing darkness to grow, but it cannot be destroyed. All you have to do to regain the light is remove whatever obscures it, be it Melkor's or Sauron's evil. The great thing is that the moment you remove whatever blocks the light, it immediately flows again, illuminating the world as brightly as before.
Of course, as The Lord of the Rings so aptly points out, removing the obstacles to the light is often very difficult and comes at a very high price. More importantly, even though the light returns as strong as before once the obstacles are removed, the damage caused by their evil lingers, sometimes long after.
Shadows of evil
Tolkien often used the word shadow in relation to evil and evil characters, even going so far as to refer to Sauron as the Shadow. As the darkness created by an object blocking out light, a shadow lacks substance. Shadow can also mean a faint representation, in the sense of "he is only a shadow of his former self." And shadow can refer to darkness and gloom, as when Aragorn tells Celeborn and Galadriel that Gandalf has fallen "into shadow" in Moria.
Tolkien's calling Sauron the Shadow is an effective way of getting across his dark, evil aspect while reinforcing the insubstantiality of his evil. Same goes for Sauron's good buddies, the Ringwraiths, or Nazgûl. The Ringwraiths, too, are just dark shapes that instill great fear, even though they seem to lack any substance. Consider that when Merry stabs the Lord of the Nazgûl, his hauberk (coat of mail) gives the only indication of where to try to wound him.
The threat of becoming a shadow like one of the Ringwraiths or Sauron himself is one that particularly menaces Frodo as the bearer of the One Ring. After the Lord of the Nazgûl wounds Frodo with the Morgul-knife, Gandalf notices that Frodo is becoming slightly transparent. As the Ring's evil power becomes stronger as Frodo nears Mordor, this process of turning into a wraith becomes more pronounced — Frodo is literally becoming a shadow.
In Frodo's struggle against turning into a wraith, Tolkien illustrates one process of becoming evil (metaphorically descending into shadow). It is not a process that happens all at once — it progresses gradually over time, like Frodo's very gradual fading. Some critics suggest that Frodo's initial disgust toward Gollum — his criticism of Bilbo's pity towards him, and his revulsion at the idea that Gollum is at heart a hobbit — are due to Frodo's fear of what he could become. In this view, the evil that Frodo carries can separate him from his own self and all he holds dear. In the end, the power of Ring, especially given Frodo's continued use of it, corrupts him and convinces him that he can stand against Sauron and be the new Lord of the Ring.
Tolkien's notion of "wraithing," that is, gradually giving yourself over to a controlling power until you're just a shadow of your former self, helpless before the evil influence, is very compelling and particularly applicable to modern life. Note that the power causing the "wraithing" doesn't have to be as symbolic as the Ring: it can be any of the thousand-and-one addicting influences that rob people of their humanity. The key idea here seems to be that this process of becoming evil renders all of its "wraiths" into indistinguishable shadows, robbing them of individuality and personality.
Tolkien and the last battle against evil
In the Norse mythology that so influenced Tolkien's writings, the final battle that ends the world is called Ragnarok, the Doom of the Gods (or Twilight of the Gods). This great conflagration not only destroys the Norse gods of the higher realm of Asgard, such as Odin and Thor, but also ends up destroying all of Midgard (our world of "Middle-earth") as well. But from the ashes of this final battle, a new, more beautiful world eventually rises.
Ragnarok is quite unlike the Christian notion of Armageddon in the Book of Revelation. There, the angels of God overwhelm and cast down the Devil and his beasts just before God creates a new heaven and earth — it's not even a fair fight. In Ragnarok, the gods face giants and other fell beasts knowing they will be killed in the process. In other words, the Norse gods face the Last Battle as any Viking hero would: ready and happy to die if necessary. In this Norse version of the end of the world, both good (the forces of world order) and evil (the forces of chaos) annihilate each other. The few survivors establish the new world.
Despite Tolkien's firm personal belief in a Day of Judgment, though, his fantasy works show no such certainty, only hinting at the possibility of a Last Battle and a new world to follow. His clearest reference is in the Dwarves' belief that at death they will wait in the Halls of Mandos until the Last Battle, at which time they will come out and help their father Aulë fashion a new world. Other than that suggestion, Tolkien gives only vague references to a day when the world is changed or made anew.
But you can imagine that if pushed to depict the Last Battle in Middle-earth, he would fashion something closer to Ragnarok than Armageddon. This assumption is based on the way in which Gandalf and Aragorn face the Battle of Morannon ("Black Gate") near the end of The Lord of the Rings. Here, they decide to face the Enemy and his great forces right outside the Black Gate of Mordor — not from any naive notions of victory but to buy necessary time for Frodo to destroy the Ring in the fires of Mount Doom.
They know full well that the chances of Frodo succeeding are remote, and that even if he does they may not live long enough to see it. Nevertheless, like the Norse gods at Ragnarok, they're ready to battle the enemy in the face of almost certain defeat. The biggest difference between the heroes of the Battle of Morannon and those of Ragnarok is that Aragorn and the others show none of the good old Norse Gods' relish for the fight and little of the Rohirrim's delight in a warrior's death.
Rather, the warriors of the Battle of Morannon seem quite resigned to their fate. They face this tremendous challenge with all the fear and trepidation of modern, non-professional soldiers. In fact, Tolkien's attitude in The Lord of the Rings towards war and battling evil is very contemporary. In most of the battles, good guys are far outnumbered by bad guys, and their chances of victory are usually slim to none. Ignoring for the moment the adolescent battle banter between Gimli and Legolas, Tolkien's warriors are serious about their jobs, especially Gandalf and Aragorn. The only exceptions seem to be the Théoden and the Rohirrim.
Tolkien's attitudes toward warfare in The Lord of the Rings closely resembles the outlook prevailing since World War I: War is hell, the world is a dangerous place, and the forces of evil are everywhere and numerous. Just as fewer of us are certain that good always triumphs over evil, heroes such as Gandalf and Aragorn are far from certain about the success of their desperate venture to prevent Sauron's total domination of Middle-earth.
Tolkien's heroes fight with little assurance of victory in their particular struggle, to say nothing of a final triumph of good over evil in a much later Last Battle. They fight knowing that they must resist evil to preserve the islands of light in Middle-earth and to arrest the spread of darkness, even with no guarantee of success. In them, you can see a blend of Viking courage with a lot of 20th-century ambiguity over the final outcome of the good/evil question.