system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why was everyone so scared of Sauron if he got taken out the first time by cutting his ring finger off?

Not trying to troll. That part always bugged me in the movie and I haven't read the books.
gpt
The movie misleads you.  In the real story, Isildur got the ring only *after* Sauron was defeated.  His physical form, the body/form he was wearing, was effectively "killed", and that's what banished him from doing anything in Middle Earth for a long time.

By the late third age, he had recovered and returned in a new form, but he still lacked a large portion of his former power because that power was in the ring which was not in his possession.  But he was still quite powerful, and he had been rebuilding his old alliances and collecting a vast army in addition.  Plus, he had nine Nazgul - wearers of the nine rings for men - as his lieutenants.  And the ring was trying to return to him, and it wasn't that unlikely that it would succeed.

Defeating him the first time was very hard.  It wasn't simply cutting off a finger like it appears in the movie.  It certainly wasn't a foregone conclusion; he could have won that war.  He lost that time, but that didn't mean he couldn't win this time.

Edit: BTW, the ring is actually why he wasn't fully defeated in the first war.  Because it contained a significant portion of his power, while it remained intact he couldn't be fully destroyed.  Because Isildur kept the ring intact, Sauron was able to continue to exist formlessly for a while, and eventually slowly rebuild his strength.