Treebeard had been friends with Saruman. It is described in The Two Towers that Saruman visited Treebeard in Fanghorn Forest and had talked with him on various subjects of concern. Saruman gathered information from Treebeard about the Forest of Fangorn; its inter-twinning paths were of particular concern to him."One felt as if there was an enormous well behind them, filled up with ages of memory and long, slow, steady thinking; but their surface was sparkling with the present: like sun shimmering on the outer leaves of a vast tree, or on the ripples of a very deep lake. I don't know but it felt as if something that grew in the ground — asleep, you might say, or just feeling itself as something between root-tip and leaf-tip, between deep earth and sky had suddenly waked up, and was considering you with the same slow care that it had given to its own inside affairs for endless years."
After Merry and Pippin's meeting with Treebeard, Treebeard called a meeting of the Entmoot, which lasted three days, who then decided to call an attack on Isengard and Saruman. Since Leaflock and Skinbark were the oldest of the Ents along with Treebeard, they refused to fight, however, Treebeard thought he was going to his doom during "the last march of the Ents." Treebeard hoped that some of the younger Ents would come instead of just the two, and, during the night he spread the word. They later launched an assault on Isengard.
After Treebeard ordered the Ents to march to Isengard, the Ents felled Saruman's walls and destroyed every object in and around Isengard; the Tower of Orthanc could not be breached, but Saruman was trapped within. Treebeard stopped the attack on the tower when he realised their efforts were in vain: the tower was too strong. The Ents were ordered to unleash the waters of the River Isen, which flooded Isengard. When Merry and Pippin departed Treebeard he requested them to watch for the Entwives. At one point in the book, Tolkien gives hints on the whereabouts of the Entwives. It is speculated that they were spotted by Samwise Gamgee's cousin Hal in the North Farthing. However, this may or may not be conjecture:
In the Years of the Trees where the Ents were thriving in 1495 Morgoth had re-established his realm in Middle-earth. With this the Entwives had moved across to the east where Anduin lay. Treebeard's Entwife Fimbrethil was driven from her land and the two were separated forever. This may have been the dominant cause of the loss of the Entwives and the loss of the Entings."But what about these Tree-men, these giants, as you might call them? They do say that one bigger than a tree was seen up away beyond the North Moors not long back......But this one was as big as an elm tree, and walking — walking seven yards to a stride, if it was an inch."
The Elven-realm Lothlórien was situated near the North of Fangorn Forest and Treebeard had met the Lord and Lady of the Galadhrim of Lothlórien, who refer to him as "Eldest". Over time the Ents and the Elves separated and the Elves had nothing more to do with the business of Ents.
The "Great Darkness" is an uncertain term used by Treebeard, apparently referring to the time that Middle-earth was under the dominion of Morgoth. It was in the Great Darkness before the Sun and Moon that Melkor first created the Orcs and the Trolls, so that these creatures feared sunlight and shunned it. The Darkness described by Treebeard was not dispelled by the coming of the Sun: Morgoth held his fortress of Angband for centuries afterwards, and even after he was overthrown, the Darkness lingered (even at the time of the War of the Ring, remnants of the Great Darkness could still be found in the deep valleys of Fangorn forest, and elsewhere).