system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why was Sauron so easy to defeat when he had the ring of power?

A sword to the finger did the trick? Why was it such a big deal if he obtained the ring again? (I only watched the movies and it has been awhile.) Let me know is this is not a fitting post for this subreddit.
gpt
Narsil (the sword of Elendil) had great power in itself; it was forged by Telchar, the greatest smith of Nogrod, in the First Age. In fact, Telchar was probably the greatest smith of all Middle Earth save Feanor (smith of the Silmarils, jewels of unimaginable beauty, and other wonders) and Celebrimbor (smith of the rings of power-the seven of the dwarves and the nine of men with the help of a disguised Sauron, and the tree elven rings on his own). So that explains the first question-it wasn't *just* a sword to the finger, it was one of the most powerful weapons in Middle Earth to the finger.

As to the second question: one of the purposes of the Rings of Power was to enhance and extend the powers of the wearer. The Nine Riders would be simple wraiths without their rings; Gandalf's ability to rouse passion and hope in the hearts of men would have been much less without Narya (corrected thanks to zedlx), the elven Ring of Fire. Sauron's ultimate goal was to dominate the other rings of power, and through them, all the peoples of Middle Earth. In order to do so, he created his own ring of power, the One Ring, by pouring his own fire and spirit into it; thus the Ring is both part of and an enlarging of his own power. With the ring, he is a force nearly invincible; remember it was only Isildur's luck and Sauron's hubris that lost him the ring in the first place. Without it, it took him hundreds of years to regain the power to merely create a body for himself. 

Regaining the ring means he becomes as powerful as before, but this time with the people of Middle Earth in disarray with petty squabbles between them. There would have been no great alliance of elves and men; only hatred, domination, cruelty, and darkness.