We as the readers know that this Aslan is false, but surely the Narnians don't know this at all other than our main protagonists.

I think that now that we know that the donkey in the lion skin was just a false, the suspense turns to the Narnians choosing who to follow. It becomes suspenseful because if the Narnians choose to follow the false Aslan... they go to an unmentioned place that does not seem good at all.

Let me think about this one for awhile for a better answer. Good question.

One thing that I think must be mentioned, and which I think is crucial to this question, is the fact that this book was written for children. As adults, we can emotionally handle a little bit of mystery; we could handle the fact that Aslan isn't behaving quite as he should, and we know to be skeptical about it.

However, if a child, who doesn't have an incredibly keen sense of logic about him yet, sees a character like Aslan start doing such terrible things, they could get very confused. Thinking back to when I was a child, if I had seen the same Aslan who created Narnia in MN, who lays down his life for the innocent in LWW, who aids Narnia in need in HAHB, and restores it to its greatness in PC, who's breath is the wind in the sails of the DT, and who's guidance is essential to SC: If I saw that lion murdering trees, wasting the forests, selling out his own people, murdering, allowing foreign occupation, etc...... I would have been emotionally scared stiff! It wouldn't feel like Narnia anymore; it would feel like a scary place where even norms cannot be trusted.

Relating this to our world, I think Lewis wanted to make it quite clear to children that this would be a false Aslan, because they knew not only what Aslan meant to Narnia, but what He means in our world too. If they saw the real Aslan behaving thus, what would stop the God of our world from doing the same.

Maybe because he wanted to establish, right from the start, that Shift was a duplicitous, blasphemous charlatan, a false prophet and make the reader hate him right away.
He is, after all, the one mainly responsible for the end of Narnia. I think Lewis probably didn't want any ambiguity about it. Shift is the villain and he wants us to identify him as such right away.

Maybe because he wanted to establish, right from the start, that Shift was a duplicitous, blasphemous charlatan, a false prophet and make the reader hate him right away.
He is, after all, the one mainly responsible for the end of Narnia. I think Lewis probably didn't want any ambiguity about it. Shift is the villain and he wants us to identify him as such right away.

I like that idea... It's certainly what happened when I read the book for the first time. You know that it's Puzzle who's pretending to be Aslan and Shift is controlling the whole thing like an evil puppet master and the whole time I found myself just shouting at the characters to get a sense that he's not the real thing. So maybe, yes?

It's also possible it was done to prevent the reader from trying to guess what is going on. If there had been Aslan doing things that the reader knows the real Aslan wouldn't do, the reader would have tried to come up with an explanation for it. I know that I would have been disappointed if I had come up with a lot of complicated schemes that would have been possible... and found out that it was just a donkey with a lion skin.

With the opening scene, the reader already knows that Shift is planning something. The surprise is that the Narnians really believe the fake Aslan and that the situation has got so bad so quickly.

I think it's because we needed to know that Puzzle was really someone with a good kind heart who truly wanted to follow Aslan - but was also very easily manipulated. Puzzle's story is a warning for those who don't want to use their own descernment.

It also lets us see later that Shift wasn't as smart as he thought he was - he becomes the puppet of Ginger and the Calormens.

A few positives did come out of this opening:

I think it gives the story an impending sense of doom. It starts the story off on an "uh oh, this can't be good.." note. It affects the tone of the story immediately.

It is also an unexpected opening. It made me wonder "hmm, who are these guys? where is the story going with this?"

And I just like the Shift and Puzzle stuff, haha. I love their relationship.

.... But an unfortunate side effect is that the mystery of the false Aslan is spoiled, and I would have loved to have been in the same boat as the characters.

I know that I would have been disappointed if I had come up with a lot of complicated schemes that would have been possible... and found out that it was just a donkey with a lion skin.

Good point. I can understand that. But I wonder if I would have enjoyed the anti-climax of that. A bit like the lack of fighting during the Lone Islands scene in VDT. The characters themselves seem to find it a bit humorous that all along, it was just a clumsy donkey in a lion skin. Either way, it is nice that when they discover the fake Aslan, that it's a character we know from the beginning of the book. That moment probably wouldn't be as powerful if it was just a random donkey being introduced for the first time.

So here is the compromise I would have proposed to Lewis: In the opening chapter, hint that Shift has devious plans for Narnia involving Puzzle, but don't say exactly what. Don't say the thing they found was a lion skin. Just say it's an odd soft yellow object, and Puzzle isn't sure what it is. That way, when we hear the rumor that Aslan is cutting down the trees, we're pretty sure something is up, but we're not sure exactly what.

I think it would be kind of difficult to explain what really happened without saying what really happened in the beginning. Lewis could have opened with Tirian and then have Puzzle explain what happened when Jill took him from the stable. But I agree, you wouldn't really understand Puzzle's character and how he really didn't want to pretend to be Aslan and only did it because he thought Aslan wanted him to dress up in a lion suit. I am trying to imagine how it would sound, and I think it would be a bit confusing.
But I do understand your point, glumPuddle, I had always wondered why Lewis didn't open LB with the children. It's the only book where kids from our world go into Narnia and it's not told from their perspective.

I think, too, that to begin with the petty domestic wrongs of the Shift-Puzzle relationship is really powerful - more so than the depiction of Evil in The Lion, the Witch and the Wardrobe, as something totally alien to the reader's life.

I think that the depiction of evil in the earlier book as clearly defined, and totally Other, is right for the target readership -- eight-year-olds? But The Last Battle is pitched to a more thoughtful, older age-group, and brings the challenges home by constructing a story showing the devastating effects of what seem like trivial self-indulgences (by both Shift and Puzzle - as parableproductions has pointed out, Puzzle's intellectual laziness is culpable, and enables Shift's self-centredness, leading to the destruction of Narnia). Starting with the mildly humorous depiction of everyday manipulativeness and moral laziness suckers the reader in for the shock of seeing what hideous results can come from 'insignificant' wrongdoing.

Caveat: I should note that Lewis doesn't thump the reader over the head with this point. The notional eight-year-old can still read the story happily, without feeling out of her/his depth.

One of the important things to the chronicles is the hope of Aslan. Reading the book as a kid, I immediately would have felt that the story was different from the other chronicles, one that felt more intense and hopeless. Something so different from the other stories, with events going from bad from worse, I probably would not want to finish the book. All of a sudden hope is replaced with doubt. With bad things happening, but everyone saying that its Aslan's orders, I would be in same boat as Tirian and Jewel- Aslan wouldn't do these sorts of things, but how can we be sure, he's not a tame lion. Opening with Shift and Puzzle changes the idea of knowing that there is a false Aslan to wondering when the real Aslan will turn up to "put things right". As the story unfolds, even when knowing there is a false Aslan, reading the book as a kid I still wondered where Aslan was and why he hadn't come to save Narnia. That's what makes the events unfolding from bad to worse all the more emotionally engaging with the finale being all the more powerful and opposite of the dread.

To put it simply: without Shift and Puzzle in the opening and knowing that there is a false Aslan, suddenly there's no hope for the story- something that would be very different for all the other Narnia chronicles. That's my take on it anyway.