Why didn't the ents play a part in the Battle of the Pelennor Fields? After they defeated Saruman?
The ents became angry at Saruman because his army cut down a large number of trees; but Saruman is an ally of Sauron. Furthermore it is known that in the 17th century of the Second Age; a vast majority of the forest was destroyed in the war between Elves and Sauron. In other words the ents had many reasons to hate Sauron. Why didn't they want to join the coalition against Sauron?
Personal Opinion:
More Canonical Answers:
Gandalf specifically asked them to stay at Isengard and make sure Saruman didn't get out.
The Ents would probably come with the Huorns, and the Huorns aren't very good at distinguishing between friends and foes. There would be a lot of friendly fire (no pun intended).
We also don't know how good the Ents are at distinguishing between Gondorians, Rohirrim, Haradrim, and Easterlings.
Even when the Ents generally accepted the need for military action against Saruman, they still had to hold a 3 day long Entmoot to finalize the plans. If they were asked to intervene in a battle against enemies who hadn't done anything to their forest, and in which many Ents would be expected to die, the Entmoot would take longer, and would be less likely to reach the conclusion the men of the west desired. In any case, the battle would be over before the Entmoot was.
The assault on Isengard and Orthanc was safer than the Battle of the Pelennor Fields- there were very few Orcs and men at Isengard to defend against the Ent attack, so casualties were very low. It was almost an assault on an undefended fortress. The Battle of the Pelennor Fields was totally different- there were tens of thousands of bad guys with axes and torches who would hack and burn the Ents without much difficulty, and huge Oliphaunts capable of knocking Ents down and tearing them to pieces, and maybe even eating parts of them. And as I mentioned above, the battlefield is riddled with trenches full of burning wood and oil - the last place an Ent would want to go. This would be a far more dangerous battle for the Ents to get involved in, and few Ents would be left alive afterwards.
As we know now, the Ents weren't needed. It would have been nice to have them around, but the men of the west did well enough without them.
As a general rule, Ents don't like men very much. They tend to focus on mankind's habit of chopping down trees and burning them or building houses out of them, which pisses Ents off. They attacked Saruman because he had destroyed the forest, not because he was killing people. They might not want to help men defend themselves from other men, since all men treat trees as commodities rather than intelligent beings "with voices of their own", as Treebeard puts it. Treebeard actually says Gandalf is the only wizard who really cares about trees, so imagine what Treebeard thinks of men. Happily, Treebeard later discovers that Aragorn cares about trees too, when Aragorn gives the Ents a large swath of woodlands to govern as they see fit. But this only happens after the Battle of the Pelennor Fields (and the Battle of the Morannon) is over.
Treebeard calls the assault on Orthanc "The Last March of the Ents". There will be no more marches, not to Minas Tirith or anywhere else.
The Ents' attack on Orthanc consisted mainly of using their root-like hands to deteriorate the stone walls surrounding the tower, then redirecting streams to flood the area within the walls. Neither of these tactics would be very useful in the Battle of the Pelennor Fields. We don't know what the Ents would bring to the table, so to speak. They might not be particularly useful for the task at hand.
Out of Universe Answer:
I maintain that it was primarily timing and logistics.
The Ents were busy defending Rohan very shortly before the Battle of the Pelennor, and things happened so quickly that they weren't able to be of any assistance:
March 11 Eastern Rohan is invaded from the North.
March 12 The Ents defeat the invaders of Rohan.
March 13 The Pelennor is overrun.
March 15 Battle of the Pelennor.
Selected excerpts from Return of the King Appendix B "The Tale of Years" (ii) The Third Age The Great Years
Consider the map below, and bear in mind that it took Théoden and company six days to march from Dunharrow (approximately the blue circle) to Minas Tirith (the red circle). Admittedly Théoden's army is much bigger than the Ent army, and the Ents can take much larger steps, but factoring in the time it would take to send them a message and it seems unlikely they could get from the northern borders of Rohan to Minas Tirith in time enough to be useful.
Don't forget that it took them a night to get from Isengard (the green circle) to Helm's Deep (the orange). It's much farther to Minas Tirith; they could probably make the trip in three days, but not when you factor in a messenger.
Of course, if I were Théoden and leading every able-bodied man into war in Gondor, I'd want to leave behind a rearguard to keep enterprising Orcs from torching my villages. It's entirely possible (though unconfirmed so far as I know) that he didn't want them at the Pelennor, because they were much more valuable to him defending Rohan while the entire Rohirrim were doing other things.
It was due to their low numbers. Treebeard mentions that there's "too few of them left" to help Gondor. They've teamed up with Gondor in the past at Dagorland against Sauron, but since they lost the Entwives they don't have the manpower that they did.
