A lot of this desert was formerly the Roman bread basket that fed the empire.Some blame goats, who crop to the ground and then dig out the roots. They also climb and denude trees. Once the green was gone, the rain from transpiration also went away.
Some blame a general climate change driven by orbital precession - if so the new trees will die out. <a href="https://en.wikipedia.org/wiki/African_humid_period" rel="nofollow">https://en.wikipedia.org/wiki/African_humid_period</a>
At some point the humid period will return - not until then will trees prosper.
That said, careful cultivation of the right species and drip irrigation with desalinated salt water powered by solar energy will be the short term path. Once built, desert solar from cells is remarkably low in maintenance cost and at the right height will allow an understory of less sun hungry plants. With an 8 foot height, and the right gaps and solar spreaders in the gaps hundreds of species are economic.
Drip irrigation has thin black plastic piping direct to roots, with a layer of black plastic under a little earth to limit loss by evaporation. Being done in arid California and Israel it works very well. Google examples.