Invasion of EregionInvasion of Eregion - With the Rings forged, both sides got what they wanted.  But they did not realize what the other had until after the job was done.

 

"Now Celebrimbor was not corrupted in heart or faith, but had accepted Sauron as what he posed to be; and when at length he discovered the existence of the One Ring he revolted against Sauron...When Sauron learned of the repentance and revolt of Celebrimbor his disguise fell and his wrath was revealed..." ~Unfinished Tales (The History of Galadriel and Celeborn)

 

When the Elves discovered Annatar was really Sauron in diguise, and Sauron learned Celebrimbor had forged Narya, Nenya & Vilya, their pseudo-alliance quickly collapsed.

 

"But the Elves were not so easily to be caught.  As soon as Sauron set the One Ring on his finger they were aware of him; and they knew him, and perceived that he would be master of them, and of all that they wrought.  Then in anger and fear they took off their rings.  But he, finding that he was betrayed and that the Elves were not deceived, was fill with wrath; and he came upon them with open war..." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

War broke out and Sauron sent legions of Orcs and Trolls to invade Eregion.

 

"...and gathering a great force he moved over Calenardhon (Rohan) to the invasion of Eriador in the year 1695." ~Unfinished Tales (The History of Galadriel and Celeborn)

 

"From that time war never ceased between Sauron and the Elves; and Eregion was laid waste, and Celebrimbor slain, and the doors of Moria shut.  In that time the stronghold and refuge of Imladris, that Men called Rivendell, was founded by Elrond Half-elven; and long it endured." ~The Silmarillion (Of the Rings of Power and the Third Age)

 

Celebrimbor, the leader of Eregion, was killed, but before that happened, Sauron tried to extract information from him.

 

"Celebrimbor, desperate, himself withstood Sauron on the steps of the great door of the Mirdain; but he was grappled and taken captive, and the House was ransacked.  There Sauron took the Nine Rings and other lesser works of the Mirdain; but the Seven and the Three he could not find.  Then Celebrimbor was put to torment, and Sauron learned from him where the Seven were bestowed.  This Celebrimbor revealed, because neither the Seven nor the Nine did he value as he valued the Three; the Seven and the Nine were made with Sauron's aid, whereas the Three were made by Celebrimbor alone, with a different power and purpose.  Concerning the Three Rings Sauron could learn nothing from Celebrimbor; and he had him put to death." ~Unfinished Tales (The History of Galadriel and Celeborn)