I had one Geography teacher who said that over the centuries people around the Sahara had chopped down all the trees for fuel and building materials, and have no cultural system of forestry management, like they have in Europe.
So, the lack of trees removes the windbreaks and the soil gets blown around, leaving roots bare and a cycle starts. Once the land is baren the warm daily updraft currents chase the clouds away so there’s no dew overnight.
Then, the reverse should happen if trees were planted at the edges and protected until they could in turn protect the next row of trees planted toward the center of the desert.
Some nations, like Swaziland, have created artificial forests where none ever grew, and turned a rocky waste into dense timber.
I once saw a picture taken in the Sahara. It was of a road in a hilly area. One side of the road was rather surprisingly vegetated, the other was completely barren. The only difference between the two? A fence keeping out the goats. There are too many people subsisting on the area, as long as there is overgrazing, reclamation cannot occur. If you remove the pressures, the land might even recover gradually on its own.
There are a few factors that contribute to the Sahara’s condition. One is in terms of weather patterns. There are these cycles of rising/sinking air called Hadley Cells (go here for an image and short discussion). As warm air rises off the equator, it cools and expands, and eventually sinks back down farther away. Major desert regions are found where the air sinks (northern Mexico and the Sahara, for instance), and tropical forests are found where the air rises (the rain forest in South America, etc). The rising air creates a low pressure, and you Weather Channel fans ought to know that low pressure means precipitation. The sinking air creates a high pressure, and thus very little precipitation reaches the ground in these areas.
As for there being surprisingly abundant vegetation in some areas: Desert soil can actually be very good for plants. They have a very high cation exchange capacity, which means positively-charged nutrients can be taken up into roots more easily. The fact that deserts are so arid means that very few nutrients are leached out by water. Of course, it also means there’s less water for the plants themselves (an unfortunate Catch-22). Additionally, deser soils tend to form a “hard pan,” which is a hard, calcified layer just under the surface. This makes growing new roots difficult.
Unfortunately, the areas of northern Africa and the Middle East had been over-farmed, so to speak. All nutritive value was sucked out, leaving a poor and barren soil. Now the conditions are so harsh that it’s next to impossible for plants to grow in some areas. Replanting efforts could be a good start, but I don’t know enough ecology yet to comment on the effectiveness of that method.
Hope I helped shed a little light on your question.