Almost all empires I've seen that existed north of Sahara, did not expand to the south. And the empires south of the Sahara did not expand to the north (except for post 15th century European colonialists, and the Kushites). Why is this? Roman Empire Umayyad Caliphate Almohad Caliphate Songhai Empire Mali Empire Kanem Bornu Empire

I understand supply lines may have been difficult to establish through Sahara interior due to lack of water, but I also know that there were several trans-Sahara trade routes going through the interior. And the corridors along the Red Sea and west Africa had sufficient water availability.

Ethiopian Trade routes Mali Trade routes Trade route 3

Africa River map

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    When you are an Empire, why would you want to expand into the big bowl of sand with no value? There are enough other targets for incorporating them into your Empire. It's a little like asking "When given a bowl of sand and a chocolate pudding, most people eat the pudding. I wonder why that is?".
    – nvoigt
    Nov 15 '20 at 10:05
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    "I understand supply lines may have been difficult ..." No, you obviously do not understand. The average size of a caravan is about 1000 camels... That is like orders of magnitude smaller than a military supply line or an army itself. Why would one bother to occupy a sea of sand bigger than most empire just to die in a week when facing the first fortified village which can lock the access to all the food and water in the whole area?
    – Greg
    Nov 15 '20 at 11:48
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    As a comparison, most slave trading roots were able to transport about 5-10 000ish slaves a year.
    – Greg
    Nov 15 '20 at 11:50
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    @Samid: "the corridors along the Red Sea and west Africa had sufficient water availability." But you cannot drink salt water.
    – Jan
    Nov 15 '20 at 15:04
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    The Roman legions were expected to live off the land while on the march. Try that in the Sahara. Nov 15 '20 at 23:07

This kind of question (why do empires not control the desert) features heavily in Ibn Khaldun's famous Muqaddimah. His conclusion is that those desert arreas are much less productive than other areas and the population is considerably harder to control. Life in those parts is tough and so people are also tough. In fact his observation is that desert tribes will often seize power for themselves if, or rather when, the (previous) ruling dynasty declines.

What is fascinating is that somewhat similar dynamics are at play on China's northern border. Many dynasties were not able to reach north of the Gobi desert. Those that were (e.g. Liao, Jin, Yuan, Qing) usually originated from north of the Great Wall themselves.

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