Battle of Gladden Fields (2 T.A.)Battle of Gladden Fields (2 T.A.) - Sauron was vanquished at the Siege of Barad-dûr (though he was not killed) and Isildur cut the Ring of Power from his hand.  He could have reduced Sauron to nothing more than a wandering spirit by destroying the ring, but he refused.

 

"The Ruling Ring passed out of the knowledge even of the Wise of that age; yet it was not unmade.  For Isildur would not surrender it to Elrond and Cirdan who stood by.  They counselled him to cast it into the fire of Orodruin nigh at hand, in which it had been forged, so that it should perish, and the power of Sauron be for ever diminished, and he should remain only as a shadow of malice in the wilderness.  But Isildur refused this counsel, saying: 'This I will have as weregild for my father's death, and my brother's.  Was it not I who dealt the Enemy his death-blow?'  And the ring that he held seemed to him exceedingly fair to look on; and he would not suffer it to be destroyed." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

But even though Sauron had been defeated, he still had agents to carry out his will.

 

"But Isildur was overwhelmed by a host of Orcs that lay in wait in the Misty Mountains; and they descended upon him at unawares in his camp between Greenwood (Mirkwood) and the Great River, nigh to Loeg Ningloron (Sindarin for), the Gladden Fields, for he was heedless and set no guard, deeming that all his foes were overthrown.  There well nigh all his people were slain, and among them were his three elder sons, Elendur, Aratan, and Ciryon; but his wife and his youngest son, Valandil, he had left in Imladris (Rivendell) when he went to the war.  Isildur himself escaped by means of the Ring, for when he wore it he was invisible to all eyes; but the Orcs hunted him by scent and slot, until he came to the River and plunged in.  There the Ring betrayed him and avenged its maker, for it slipped from his finger as he swam, and it was lost in the water.  Then the Orcs saw him as he laboured in the stream, and they shot him with many arrows, and that was his end." ~The Silmarillion (Of the Rings of Power and the Third Age)