First Siege of Imladris

In T.A. 1699-1700, Sauron's forces overwhelmed Eriador. Gil-Galad had sent Elrond to reinforce Celebrimbor in Eregion, and Elrond too would have fallen if not for an attack on Sauron's rearguard by Durin III and all of the Khazad-Dum Dwarfs. This allowed Elrond to escape with a good portion of elves and men--with them, he founded the refuge of Imladris with Celeborn who had come to aid Eregion as well. This siege lasted after the Battle of the Gwathlo, and the combined armies of Gil-Galad, Elrond, Celeborn, and Ciryatur utterly destroyed Sauron's forces, thus ending the siege.