The state of Imladris during the War of the Ring

I've heard it many times refered to, and the passage is said to be somewhere in Return of the King, but for the life of my I can not find it - evidence that there was a siege of Rivendell, and that was the apparent reason why Elrond could spare not but his sons, and the Grey Company. I know Rhovanion was beseiged, and Legolas refers to this in "The Passing of the Grey Company", as vague as elves do.

Do any of you know of this said attack upon Rivendell. I'm quite vexed by it.