History: division and happy endings
One very odd thing about Shakespeare's Lear is that all his sources tell the story with a happy ending: Lear is restored to his throne. It is only by a rather arbitrary twist of the plot, as Edmund sends orders to kill Lear and Cordelia, that the tragic ending* is assured.
Despite the optimism of the sources, Shakespeare's audience would have anticipated a tragic outcome from the opening scene where the kingdom is divided: though Lear specifically wishes "that future strife / May be prevented now" (1.1.46-47), any division of the kingdom would have been seen as a recipe for civil war*.
Footnotes
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O happy defeat
Though Lear's initial response to the loss of the battle is optimistic: he delights in the freedom from Court politics that the prison will offer.
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Carving up the kingdom
Compare the scene in Henry IV, Part One, where the rebels, headed by Hotspur, similarly plan to divide the kingdom, and immediately begin to argue.