The Elves come to King Théoden’s aid at the battle of Helm’s Deep.
In Peter Jackson’s film The Lord of the Rings: The Two Towers, an Elven force from Lothlórien arrives at Helm’s Deep shortly before Saruman’s army attacks. The Elves are led by Haldir, who tells Théoden that they have come to honor an old alliance between Elves and Men. Their presence gives the defenders additional skilled archers and reinforces the fortress during the desperate siege.
This detail is one of the major differences between the film and Tolkien’s novel. In the book, no army of Lothlórien Elves fights at Helm’s Deep. Instead, the defenders are Rohirrim and other Men, and the battle’s later turning point involves Gandalf, Erkenbrand, and the arrival of reinforcements.
The Ents are involved elsewhere in the same story, attacking Isengard after Merry and Pippin persuade Treebeard to act. The Dwarves do not participate in the defense of Helm’s Deep.