Temple of Doom was the second Indiana Jones movie, but it was set before Raiders of the Lost Ark. Why was the decision made to set the second movie before the first? I didn't notice any story points that wouldn't have worked just as well set after Raiders.
Lucas made the film a prequel as he did not want the Nazis to be the villains once more.
From Lucas himself:
Lucas: We made Temple Of Doom a prequel because we didn't want to use the same bad guys. We had ideas about the Monkey King. We had ideas for a haunted-castle movie but then Steve had just done Poltergeist and said, "I don't wanna do that again." We were struggling to come up with another MacGuffin. We couldn't find anything as good as the Lost Ark. We ended up with the Sankara Stones, which was a little obscure.
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