In the chapter "A Journey in the Dark" of The Fellowship of the Ring, the Fellowship was attacked by Sauron's Wargs, evil wolves, in Hollin, after their failed attempt to cross the mountains above Moria. The battle was the first taste of action the Fellowship had on their journey. Both Boromir and Aragorn each killed at least one warg; Legolas killed four. Gimli did his part as well, though the book does not specify whether or not he slew any wargs. Gandalf set the nearby trees aflame, which drove the wargs away. The bodies of the dead wargs were gone the next morning; they were no ordinary wolves.
In the chapter "The Bridge of Khazad-dûm" of The Fellowship of the Ring, the Fellowship is attacked by Orcs in the Chamber of Mazarbul (Chamber of Records) in Moria.
Pippin had foolishly dropped a stone into a well in a guard-room some days earlier, and it is probable that the Orcs were alerted to the intruders' presence. These Orcs had previously fought and wiped out an expedition of Dwarves led by Balin, who had set off to re-colonize the mines, long-abandoned by their folk. The Chamber of Mazarbul contained Balin's tomb.
The attackers included both common Orcs and larger black Uruks of Mordor — the first time the Uruk-hai are mentioned in the book (though the term "Uruk-hai" first appears in The Two Towers). The Fellowship tried to bar the doors to the chamber on the attackers' side (there were two doorways) with broken weapons, but a cave-troll beat on the doors and forced a foot and a scaly arm through.
Boromir hacked at the arm, but only notched his sword. Frodo pierced the foot with Sting and drove the troll off. The Orcs loosed arrows through holes made in the doors, hitting no one. At last, the orcs managed to break their way through the doors, and a fierce skirmish ensued.
When the Fellowship had downed 13 Orcs, the rest fled outside; then a large Orc-chieftain charged, pinning Frodo to the wall with a spear. Aragorn killed the chieftain and carried Frodo away as they left the chamber through the doors on their side, thinking him dead. It is later revealed that Frodo was only bruised, thanks to his hidden mithril chainmail.
Gandalf remained in the chamber while the rest of the Fellowship fled, and set a shutting-spell on the other door. But then a mysterious creature arrived and put a counter-spell on the doors, making them open. Gandalf then used a "word of Command" to shut the doors, but they broke under the magical strain, and the Chamber collapsed. Gandalf only just escaped, and said to the others that he had met his match.
In Peter Jackson's 2001 live-action version of The Fellowship of the Ring, it is enlarged; the Fellowship fights more Orcs and the cave-troll, who spears Frodo. The Balrog is introduced later, when the heroes are surrounded by Orcs after fleeing the chamber. The setup is also modified. Pippin drops a skeleton, bucket and a chain into a well in the chamber itself; almost immediately, Orcs attack.
After Gandalf and the Balrog fell into the abyss in Moria on January 15, 3019, they fought long underground and then climbed the Endless Stair to the peak of the Silvertine where Durin's Tower stood. There on a narrow eyrie they fought the Battle of the Peak from January 23 to January 25. The Balrog, whose fire had been quenched in a subterranean lake, burst into renewed flame. The sun shone at first, but as the combatants struggled thunder boomed and lightning flashed, vapour and steam rose, and ice fell. From a distance it appeared that a storm was raging over the Silvertine. At last, Gandalf defeated the Balrog; he threw the Balrog down from the peak, and the mountainside broke where the Balrog landed. Gandalf then died from his wounds and passed into darkness out of thought and time, but Eru Ilúvatar sent him back to Middle-earth as Gandalf the White to complete his task. When Gandalf awoke on February 14, he was lying naked on the peak of the Silvertine. Durin's Tower had crumbled and the entrance to the Endless Stair was blocked, and Gandalf was trapped high on the mountaintop until Gwaihir the Windlord came on February 17 and bore him to Lothlórien.
The Two Towers starts with fighting upon Amon Hen, where the Fellowship of the Ring is attacked by Uruk-hai and other Orcs. It directly continues from the last chapter of the first volume, The Fellowship of the Ring. The fighting is not directly shown, being narrated by various characters instead in the chapters "The Departure of Boromir" and "The Uruk-hai".
The Uruk-hai and some of the other Orcs were either in the service of Sauron or Saruman, while other Orcs had been pursuing the Fellowship since their sojourn through the Mines of Moria, to avenge their slain fellows. They had already been stalking the company for some time before the attack, and had previously shot arrows at them as they passed down the River Anduin.
Previously (in The Fellowship of the Ring), the Hobbit Frodo Baggins had resolved to continue the quest of destroying the One Ring of Sauron alone, due to its influence having grown on Boromir from the realm of Gondor, who wanted to claim it for his country and for himself. Frodo had put the Ring on,(functionally making him invisible,) to get away from Boromir and had gone missing. The rest of the company had grown anxious and had scattered, looking and calling for him. Aragorn, who led the company after Gandalf's fall in Moria, had ordered Boromir to watch over Merry and Pippin, two younger hobbits. However, Frodo's faithful servant Samwise Gamgee espied his escape, and insisted on coming with him to Mordor, land of Sauron.
Misfortune befell Merry and Pippin, for they blundered upon the Orcs. Sauron and Saruman both desired to gain (or in the former case, regain) the One Ring, and had instructed the Orcs to take the hobbits alive without taking spoils, but to kill the rest. Thus the Orcs surrounded the two and only tried to grab them, even after Merry had cut off some of their hands and arms. Then Boromir came and scattered the Orcs, and the three ran away. Saruman's Uruk-hai, led by Uglúk, then attacked and shot Boromir with many arrows. Boromir blew his horn, calling for aid, but it came too late, for the hobbits were captured. The Orcs cut his horn in half and left him lying against a great tree, mortally wounded.
Meanwhile, the elf Legolas and the dwarf Gimli were in the forest, hunting Orcs by themselves. Aragorn had sat on the Seat of Seeing, from which Amon Hen got its name, looking for Frodo, but had not seen him. However, he heard Boromir's horn-calls, and rushed through the trees. He came too late, for the Orcs were gone, and the hobbits with them, and Boromir lay dying, surrounded by at least 20 orcs that had died by his hand. Before he expired, Boromir informed Aragorn of the situation, and charged him to save Minas Tirith from falling, as he had failed it.
Legolas and Gimli then turned up. The three remaining members inspected the orc corpses, laid Boromir to rest in a funeral boat, and paid their last respects to him through a mourning song. They sent the boat down the Falls of Rauros, but it was discovered intact by Faramir, brother to the deceased, three days later.
In Peter Jackson's 2001 live-action version of The Fellowship of the Ring, it is shown as a climactic battle, where all members of the Company except Sam and Frodo slay several of the creatures. The Orcs are solely composed of Uruk-hai, and their leader Lurtz, an original character, mortally wounds Boromir, though not before Boromir slays dozens of orcs. Lurtz is the Uruk to slay Boromir, shooting him with three arrows from a longbow, but before he is able to shoot him again, Aragorn appears and, after a duel, kills Lurtz. The other Uruks have all departed by this point. The dying Boromir hails Aragorn as his "brother, captain and king", which his book counterpart does not do. In the extended edition of The Two Towers, the Uruk-hai meet with Mordor Orcs who wish to take them to Sauron.
It was fought by Men against Men: Rangers of Ithilien, led by Faramir of Gondor, against Southrons of Harad, Men allied with the evil Sauron. Most of the fighting is actually not seen, but heard by the hobbits Frodo Baggins and Samwise Gamgee.
Frodo and Sam had come across the Rangers prior to the fighting; their guide Gollum had sneaked off. While Faramir left two of his men, Mablung and Damrod, to guard them, the Rangers ambushed the Haradrim, who were soundly beaten. As the battle ended, Sam had a flash of empathy for a dead warrior of Harad who had fallen at his feet; he wondered if he was truly evil at heart, or merely deceived or enslaved by Sauron. Immediately afterward, he saw a raging, out-of-control Oliphaunt, a great beast of war he knew only from folklore (which the author describes as a larger ancestor of the modern elephant), and was both elated and terrified.
There were ~ 200-300 Rangers of Ithilien left following the battle, and the Southron regiment was defeated and most were slain.
After Faramir questioned Frodo for a while, the Rangers withdrew to their hidden stronghold, Henneth Annûn, taking the hobbits with them.
The extended DVD version of Jackson's film includes the sympathy for the dead man of Harad, though it is Faramir who says it.
In The Return of the King the Orcs of the Tower of Cirith Ungol, originally 4,000 Orcs and 1,000 Uruks, but (due to the man-power needs of Minas Morgul) are now 1,500 Orcs and 1,000 Uruks (500 of the Orcs being outside, on patrol) fight over the captured Frodo's mithril shirt, and nearly wipe themselves out. The two sides were led by Shagrat, Captain of Cirith Ungol, and Gorbag, a minor commander from Minas Morgul, respectively an Orc and an Uruk. Shagrat wished to follow his orders, which were to send EVERYTHING found with captive to Lugburz (Barad-dûr), while Gorbag coveted the mithril shirt for himself. Also, we may assume some over-reaction on the part of the Orcs, seeing their leader, Shagrat, a lesser Orc such as themselves, being challenged by a tall, large, muscular Uruk (a breed of Orc that dominated society in the other areas of Mordor) As a result, this tense argument transformed into a xenophobic crusade on the Uruks, resulting in there being now only a handful of Orcs left alive in the tower, making it much easier for Sam to break in and rescue Frodo. In the book, Shagrat is the only survivor, after taking Frodo's posesessions to Sauron he is killed by Sauron.
In Orthanc, Saruman overpowers Gandalf in a magical duel involving telekinesis, which is not present in the book.
Jackson also changes the encounter with the Black Riders on Weathertop. Whereas in the book the Riders quickly retreat after their leader stabs Frodo with a Morgul-blade and Aragorn chases them away with burning brands, the film has an extended swordfight between Aragorn and the wraiths, in which some of them are set on fire.
Similarly, the encounter with the Watcher in the Water is also expanded. In the book, only Sam hacks at the creature's tentacle and its form is not shown in its entirety. In the film, it is interpreted as a many-tentacled monster which attacks the whole Fellowship. Ralph Bakshi also expands on this scene a little by having Boromir fight the Watcher who is similarly interpreted, after Sam.
Jackson also inserts an attack on the Rohirrim going to Helm's Deep by Saruman's Warg-riding Orcs in The Lord of the Rings: The Two Towers.