[Tuor in Gondolin]

And in a way Paris was between a rock, a rock, and a
hard place- what with the three vain goddesses starting
the whole thing. It is interesting that the theme of fated
destiny in the Trojan War (essentially effecting the actions of
both humans and gods) is only (I believe) directly used by
Tolkien with the Children of Hurin, which makes it somewhat
discordant to the general ethos of Middle-earth.

Elsewhere in the tales he seems, to me, to effectively
combine free will and Iluvatar seeing that his plan
for Middle-earth and its peoples, by allowing evil to have
the possibility to "win" for a time but eventually redress
a given situation (for example, Morgoth having a nice winning
streak in Beleriand). And I think somewhere Gandalf muses
that he will not have totally failed if anything fair lives in
Middle-earth (or something to that effect).