We know as a matter of historical fact that LOTR is heavily inflected by Tolkien's experience fighting in World War I. It was not, nor was it intended to be, an allegorical work. At the same time, his experiences were very influential. There's been a lot of ink spilled on that topic, and I've not read most of it, but even at a glance, one can see the reflections and resonances.
Jackson, in his treatment of LOTR, makes two very consequential and contra textual changes to Tolkien's work. The first is the way he treats the One Ring. It is, in the movies, an absolute corrupter. It deprives those near it of agency. It is how we get to see Boromir as a tragic hero, and weep for him. In the end, it's not really his fault that he tried to rape away the ring from Frodo, he was caught in the thrall of the One Ring, and it wasn't really his fault. This reading of the power of the ring also forces Jackson to do the most egregious bit of rewriting. Faramir in the book said, "I would not take this thing, if it lay by the highway. Not were Minas Tirith falling in ruin and I alone could save her, so, using the weapon of the Dark Lord for her good and my glory." In the movie he is strongly tempted by the ring, and almost steals it from Frodo. That impeachment of his character still infuriates me. That change also alters our understanding of Denethor, makes his choices and tragedy less explicable. In the book, we understand Denethor through his relationship to his sons. This change makes Denethor much more of a cipher, makes his choices less explicable. The weight of that change steals agency from every character who interacts with the ring. It's subtle, but that choice changes the heroics of Sam, the temptation of Galadriel, the actions of Gandalf in ways that make their choices less consequential and more fated.
The other extremely consequential change, which
In Jackson's treatment of LOTR, the hobbits' context is largely played for a joke. Their heritage, their concerns, their delights in food and drink are reduced to jokes about second breakfast and the smoking scene after the flooding of Isengard. None of these things are real for Jackson. He doesn't care. The fact that who they are is partly a product of where they came from, that the choices that they make are heavily inflected by their past, is stripped away.
So, in LOTR, we see Jackson strip his characters of agency, and treat their context as a joke. He doesn't manage to do this as thoroughly in LOTR as he does in TSNGO, because the story Tolkien told doesn't really let him completely denude the characters of choice and past.
I only saw the first of The Hobbit trilogy. I reacted to it like blue food. I had an extremely bad reaction to Radagast the Brown. I found him vastly offensive. But I would also argue that Bilbo can't carry the story that Jackson said he wanted to tell. He's a simple hobbit with simple desires, and that is the fundamental charm and strength of Tolkien's story telling. We see small people as consequential. Not because they are huge heroes, but because they want a nice cup of tea, and yet still are endowed with great courage. This huge scope robs Bilbo of agency. He's a bit part in a large play, yes. But what he does matters a lot, and he does it for his own reasons, within his own frame of reference. Jackson doesn't seem to think that this frame of reference matters. He doesn't seem to believe that people's actions actually matter.
Tolkien, I think, fundamentally believed that people matter. Their choices matter. Their lives matter. The world is huge and strange and there is a larger story than any of us can comprehend of which we are a part. But we are not an inconsequential part. We matter. Our lives matter, our past matters, our choices matter, and our future is not completely outside our control. No one wants to live in such terrible times, but, "that is not for them to decide. All we have to decide is what to do with the time that is given us."
I think that Jackson has a broken sense of story. I think that he has a broken sense of agency, of consequence, of truth. I think that shows in his treatment both of LOTR and the Great War.