system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] What caused to Saruman lose hope in the fight against Sauron enough to join him?

Even after Saruman's defection, defeating Sauron was obviously a real possibility (after all they did win), before his defection it must of been even less obvious who would win.  Even without taking into account the role of the actual people on the ground and involved directly in the conflict - the combined forces of most of the supernatural beings in the universe were against Sauron, surely leading Frodo and giving him strength.  God itself was against Sauron even if not interfering directly, why would Saruman waver in his confidence?
gpt
Divine intervention in Middle-earth tended to be limited and indirect after the 1st Age: the Valar worked only through emissaries (most notably the Wizards themselves), and the number of times Eru interfered in a blatantly obvious way can be counted on one hand.

And besides, Saruman was not truly on Sauron's side: he wanted the One Ring for himself, to master it and overthrow Sauron (before taking over all of Middle-earth, "deploring maybe evils done by the way, but approving the high and ultimate purpose: Knowledge, Rule, Order"). Pretending to be on Sauron's side was convenient because Sauron did seem overwhelmingly likely to win. Sauron controlled the entire East besides Mordor and Harad, and he was growing stronger yearly. Meanwhile Gondor was getting weaker and weaker while the Elves continued to sail west in large numbers. It was becoming increasingly clear military victory against Sauron was simply not possible. And in the end it was impossible: without Frodo and Sam, Sauron would have eventually crushed all opposition.