system
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human
[Star Wars] What if Luke persuaded Vader to join *him* on Cloud City?

Suppose that Vader makes Luke an offer to join him and destroy the Emperor, and Luke makes a huge leap forward in maturity and counteroffers that Vader should join *him*, and they can work together toward the same goal. Vader thinks about it, decides that he hates the Emperor's lousy guts, and the Empire seems to be a highly unstable institution anyway, and then decides to join Luke and the Rebellion to try to overthrow the Emperor.

How does the rest of the saga play out now?
gpt
Two things were necessary for the "best win" condition on Return of the Jedi: Luke had to throw down his light saber, deciding that he would not turn to the Dark side even if it meant saving his own life, and Vader needed to realize that in killing Luke, he was finally and forever cementing the mistakes he made in Revenge of the Sith.

If Luke and Vader join forces on Cloud City, neither of those happen, and it probably dooms the galaxy.

Luke was walking a tightrope between the Light and Dark sides. When he entered Jabba's palace, he was dressed like a Sith, not a Jedi, and when he fought, he used techniques, like Force Choke, that other Jedi had not. He thought that his righteous end - save his friends, defeat the Emperor - were more important than his means.

With Vader on his team, this would be amplified by an absurd degree. This is a guy who slaughtered an entire temple full of children on the off chance it might save his wife's life. Vader would do *anything* to accomplish his goal, and with that kind of example, Luke would quickly spiral down the path of the Dark Side of the Force.

Vader wasn't redeemed until he saw his son, with a mechanical hand, being tortured by Palpatine, and realized that history was repeating itself. Vader *hated* himself, but didn't think there was a way to be redeemed. When he realized that he could sacrifice his own life to save Luke, that was when Vader died and Anakin was reborn.

But if Vader doesn't have to make that choice, he is never brought back to the Light. So sure, Luke and Vader together are powerful enough to overthrow the Empire... but they're also powerful enough to build a new Empire in their own image. The end result is that the two most powerful Force users in the Galaxy are now both fully committed to the Dark Side, and there's nobody with even a sliver of a chance at stopping them.