Strangely enough, this is an actual quote from the book: '
Of course, it is likely enough, my friends,' he [Treebeard] said slowly, 'likely enough that we are going to our doom: the last march of the Ents.
But if we stayed home and did nothing, doom would find us anyway, sooner or later.
That thought has long been growing in our hearts; and that is why we are marching now.
It was not a hasty resolve.
Now at least the last march of the Ents may be worth a song.'
In the book, though, the Ents were going into battle against Saruman himself:
the Orcs and Men had marched away to Helm's Deep by the time they attacked Isengard.
But Treebeard knew well the power of Saruman, amd was anticipating the worst: he hadn't realized how much Saruman now depended on his armies and his machinery, and he had good reason to fear.
As it turned out, Saruman had - as Pippin pointed out - totally left the Ents out of his calculations.
Still, he was able to inflict some damage - Pippin again: 'Several of the Ents got scorched and blistered.
One of them, Beechbone
I think he was called, a very tall handsome Ent, got caught in a spray of liquid fire and burned like a torch: a horrible sight.'
scifi.stackexchange.com/users/969/Daniel Roseman
While @DanielRoseman has it largely right, I think there was a sense that the Ents were dying out, and that this was going to be their last real involvement with other beings.
They were going to march on Saruman, and probably not all survive ( and they didn't ) -
although I think they anticipated their losses as being higher than they were.
But then they retreated to the woods, and died out soon after - soon in Ent terms at least, which may have been hundreds of years.
They lost some of their youngsters, and without the Entwives, there was never going to be any more.
I think they realised that this was their last chance to do something before they passed out of memory.
So it probably was their doom, but only when you look at it from an Entish perspective, not a human one.
They probably barely survived another millenia after that, hardly worth talking about.
scifi.stackexchange.com/users/3074/Schroedingers Cat
Treebeard is trying to convey that going against Saruman is the Ent's destined role, or their purpose, in the War of the Ring.
Tolkien often uses more archaic definitions for words.
In this case doom most likely means destiny, fate or some kind of end, but not always a tragic end.
scifi.stackexchange.com/users/148/TGnat
Perhaps worth to note that in the books, the whole fight of the Ents is skipped, we only hear of it after it's all over.
I guess that the Ents had pretty much resigned by the time the Hobbits entered the forest.
Saruman had pushed them back for decades and they had obviously very little hope or interest on what was going on outside their forest.
They were not aware of their strength, and Saruman being completely off-guard surely helped.
scifi.stackexchange.com/users/6778/linac
Since Tolkien sometimes uses doom in the old sense of fate or judgement or legal sentence, perhaps Treebeard partially meant that the Ents might be judged for reward or punishment when they died, however soon or late that might be, based on how they reacted to the threat of Saruman and if they helped the other free peoples in their time of need.
He might have meant that in addition to meaning he expected that it was probable that the Ents would all be killed.
Why should Tolkien write two sentences with two meanings when he could write one sentence with two meanings?
scifi.stackexchange.com/users/43538/M. A. Golding
The Ents going to war uses up a lot of their energy reserves - reserves that they hoarded for a VERY long time.
How would these Ents be able to replenish their energy and life stores back to what they used to be?
Compound this with the lack of Entwives and the consequential lack of offspring Ents - a key point for this discussion
, I think, the march of the Ents really could be seen as a hastening of their future doom - the impending doom related to their extinction as a race.
On page 958 in my paperback Return of the King , Treebeard says: 'Forests may grow,' he said. '
Woods may spread.
But not Ents.
There are no Entings.'
At some other earlier point, this urgency is also reflected when he encourages Legolas to return to Fanghorn Forest for a visit before too much time passes.
scifi.stackexchange.com/users/28107/Aith
While I would normally agree that Tolkien frequently uses the word "doom" in the archaic sense, meaning something like "destiny, judgement, fate" etc, I don't think that is the case in this particular instance.
I think Treebeard genuinely believed that he and the other Ents were going to be killed in the battle.
But another peculiarity of Tolkien comes into play here
: We generally hear what the individual characters think about things, and almost never what Tolkien, as the narrator and "voice of god", thinks about things.
This is an example of Treebeard saying what he believes, but being wrong.
It was, of course, quite possible that the Ents wouldn't have done so well in the battle, and that many of them-
perhaps even all of them - would have died.
The Ents are certainly powerful, but a wizard is an extremely formidable adversary, even for the Ents.
Treebeard was wrong about going to his doom, but I'm sure he was happy to be proven wrong.
scifi.stackexchange.com/users/44025/Wad Cheber stands with Monica
When the Ents marched on Isengard in the movie version of  The Two Towers , Treebeard said that "it is likely that we go to our doom". In the battle itself, however, the Ents seemed to do better than anyone else. They didn't appear to be anywhere near their doom.  Did I miss something in the movie? Is this a quote from the book where things go differently? Or did Treebeard simply misjudge or misrepresent their chances of surviving?
Why was it likely that the Ents went to their doom?
scifi.stackexchange.com
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scifi.stackexchange.com/questions/18020
scifi.stackexchange.com/users/6770/Jakob
 Treebeard is trying to convey that going against Saruman is the Ent's destined role, or their purpose, in the War of the Ring.  Since Tolkien sometimes uses doom in the old sense of fate or judgement or legal sentence, perhaps Treebeard partially meant that the Ents might be judged for reward or punishment when they died, however soon or late that might be, based on how they reacted to the threat of Saruman and if they helped the other free peoples in their time of need.  Treebeard may have genuinely believed that he and the other Ents were going to be killed in the battle. Treebeard suspects the Ents are going to die in battle.
Treebeard suspects the Ents are going to die in battle.