|
Arwen did not defeat the Nazgùl, her father and Gandalf produced the flood in the book. And the flood only took away their horses and their bodies, it did not defeat them. As to the other question: at the end of the Second Age of the World when the Last Alliance of Elves and Men defeated Sauron they thought he was not going to return. Only Elrond knew he was going to be back.
When the Council of the Wise was founded they knew Sauron was still alive, but they did not know where he was. It was between the time of the Hobbit and that of the beginning of LotR that the Council finally attacked the Necromancer of Dol Guldur and it turned out he was Sauron yet he escaped and went to Mordor which had been prepared by his lieutenants.
But let's suppose that Sauron was defeated by all the forces of Middle Earth. Gandalf says in the book that if the ring still survived it would be impossible to destroy Sauron. So if they defeated him in battle he would have just again gone into hiding and schemed his plans somewhere else.
Then again, not even all the "good" forces of Middle Earth combined could have breached the Black Gate by force.
|