And it slips through our fingers

[Guinevere]

Very good points, Formendacil , Alatar and Elianna!

I would particularly like to know if this theme is ultimately religious

Here is what Tolkien wrote in letter #195

Actually I am a Christian, and indeed a Roman Catholic, so that I do not expect "history" to be anything but a "long defeat" - though it contains (and in legend may contain more clearly and movingly) some samples or glimpses of final victory.

I rather share his pessimistic view. But it is not without hope - although "hope without guarantees" (to use Tolkien's expression) - and the hope has its foundation in his religion, I guess.

originally posted by Alatar:
If Tolkien were writing a pseudo-history, well, then, he knew where the story had to end, which would be with us and our world as it is in its present state.

I think that that is just what he did! (I could dig out some quotations from the letters to prove it...)