The topography of Afghanistan is worse; the terrain is more rugged, mountains are higher, valleys are narrower, and the existing infrastructure (in 2001) was severely degraded compared to Iran's. The weather is also harsher in Afghanistan, especially from Autumn to Spring. But US got in anyway and occupied it for 20 years. Although the US did not have a good time, they still wrecked Afghanistan for decades to come. I think it is doable in Iran; not saying it is easy (especially as Iran is more developed as a fighting force, but maybe not as experienced at resistance as the Afghan veterans of the civil war and preceding Soviet War).
The most important area to occupy is this flat land where majority of the oil is.
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I suspect US wants to break this piece off just as Kuwait was once splintered from Iraq. The demographics of this area - Arabs and Lurs - have a history of conflict with the state, as well as some active separatist groups (Lurs are often labeled as Kurds),
This map was suggested once by an American decades ago. It probably won't happen but you can guess what the thought process is in Washington,
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