Why is sahara considered a natural boundary?

The Sahara Desert forms a natural boundary for many countries. In particular, it forms the northern boundary of the West African states, and provides a boundary for Egypt. The Atlas Mountains provide a boundary between Morocco and the rest of Africa. 

Beyond this, the natural boundaries have little to do with Africa's current political boundaries. That is because the colonial powers, when they took over Africa, set their boundaries over these natural boundaries; to deliberately divide people of the same nationalities. 

If the Sahara was grassland, of course more North African people would live there, reducing the population density of the current North African states.

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