civil wars is your answer.
Civil wars have almost no connection with desertification.
1. lack of rain: because there is no more water flowing inside, it leads to the plants dying. Without anchoring roots, the top soil is blown away, exposing inner soil, which, when baked by the sun, can turn into a desert
2. too many cattle: Like the above, when the plants are eaten through grazings of cattles and other livestocks, the soil would have the same effect as "lack of rain".
3.a need for more food: Because there would be less virgin land, and more land is needed, the land cannot be laid fallow, and so all nutrients would be used, leading to desertification.
hope this helps
