The Ents attack Saruman after discovering that much of their forest has been destroyed.
In The Lord of the Rings: The Two Towers, Treebeard and the Ents initially decide that the conflict between good and evil is not their concern. Their position changes when Treebeard sees the devastation caused by Saruman’s industrial expansion around Isengard. Saruman has cut down large areas of Fangorn Forest to fuel his war machine and manufacture weapons.
The Ents’ attack on Isengard is therefore both a military assault and an act of ecological revenge. They break the Ring of Isengard’s defenses, flood the fortress with diverted water, and destroy Saruman’s industrial facilities. Their participation is decisive because Saruman’s army has been built around Isengard’s manufacturing power.
The Ents do not attack Sauron directly in this scene, which is a common mix-up. Saruman is the immediate enemy responsible for the destruction they witness. The sequence also gives dramatic meaning to Treebeard’s warning that no one should interfere with the Ents, who are among Middle-earth’s oldest living beings.