Like many of us, I find the Tolkien legendarium hard to accept as "just fiction". It is somehow so compelling that it just seems wrong to dismiss it as such. But we know that a man in the 20th century wrote it, and without any of the usual means of knowing that something actually happened; and while Tolkien characterizes his work markedly differently from pure fiction, he never actually says "The events of The Lord of the Rings are historical fact" or anything to that effect. How are we to reconcile these--not exactly conflicting, but certainly confusing--perspectives?
One method which has been brought up is the suspension of disbelief. This involves knowing that what one is reading is untrue, but letting oneself be influenced by it as if it were true, as nearly as I can summarize it. Perhaps Orwell said it better when he described doublethink as "The power of holding two contradictory beliefs in one's mind simultaneously, and accepting both of them."
But this isn't really what we're after, is it? Suspension of disbelief, as well as doublethink in general, requires the beliefs to be contradictory. To label the legendarium as contradictory to reality would be to deny all of Tolkien's work in creating, or finding, intricate links between it and known history. But to proclaim it historical fact would be elevating the secondary reality to a par with the primary reality. As a sub-creation, the legendarium is properly part of the primary reality, but not equal to it.
Then if we can't believe or disbelieve the legendarium, what are we to do? I think there is a middle ground--a golden mean--between illogical literal belief and cynical disbelief. In this golden mean, we believe it as we believe a legend. Now, you may say that we actually don't believe legends. True, we (probably) don't believe that when it storms, Zeus hurls thunderbolts down from Olympos. Similarly, we probably don't believe that there is an invisible Tol Eressea somewhere in Middle-geostationary orbit, directly on the tangent line which touches the Earth just over sunken Beleriand. For one thing, those are more like myths than legends; and myths are a step closer to pure Faerie than are legends. Myths aside, though, we do (or you should, if you don't) believe that there was a real Trojan War, with a real Achilles and a real Hector. It didn't likely happen exactly as told in the Iliad, but there is historical truth there, made eternal and mixed inextricably with Faerie by the great elf-friend, Homer.
With the legendarium, Tolkien has done something similar but, in the sense that he was enormously farther from the events he depicted, much harder. (No disrespect meant to Homer, of course.) He has inferred the proto-forms of many of our oldest words, traditions, and qualities; but that per se is not enough. With such a thing, as with the Trojan War, the history will always be uncertain. So what was he to do? Publish a scholarly paper saying, "Well, I think there was a historical source for the name Earendel, and he was a pretty cool guy"? Of course not. If he did such a thing, he would be looking from primary reality to primary reality at something dimly seen through the clouded glass of centuries--millennia--of history. What he did, both to gain a better view of the past, and to make it eternal, was to bypass the intervening history and look at the past from timeless Faerie, where all times...or at least, all past times...are equally represented. In a sense, he stepped to the side, out of the line of the real world, and triangulated from there to the distant past. Like Frodo with the Morgul-wound, the past that was nearly forgotten could only be healed by a trip to the Undying Lands.
So am I saying that Tolkien's view from Faerie allowed him to see the events of the past, and that everything in the books happened in our world? Well...perhaps as much as the events of the Iliad happened during the Trojan War. Remember that Tolkien delved deep into Faerie to gain what knowledge and wisdom he might. As Frodo could not return to the Shire from his voyage west, so the stories Tolkien wrote cannot be separated from their fey elements. If one tried, they would merely revert to the "scholarly paper" suggested above. They must remain as legends in the (appropriately-titled) legendarium. But I do not say that to dismiss them. They are as real an account of the distant past of humanity as we are ever going to get, barring some unbelievable technology which the Notion Clubbers would certainly rankle at. If of a history we say "This did happen", of bog-standard fantasy fiction we say "This didn't happen", and of historical fiction "It could have happened this way, but it didn't"; then of the legendarium we must say, "This is a legend, but one that like many legends is in its essence true. Though most legends admix Faerie with history over time, yet this one was lost for so long that when it returned it was already equal parts history and Faerie. The two parts are inseparable and equally true."