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Iran wants two things, to hedge on Afghanistans future by building ties on both sides of the civil war and to drive up U.S. costs to get U.S. forces off one of its borders. Geography ensures that Iran will always have an interest in Afghanistan, but Irans support for the Taliban is largely driven by antagonism with the U.S. So a U.S. withdrawal from Afghanistan would likely lessen, if not end, Irans support for the Taliban.