Food scarcity as a trigger for civil unrest

Throughout history, there have been periods of civil unrest. From religious wars to wars of conquest, historians believe that the causes of wars are closely related with religious, political or economic reasons. But when we discuss about civil unrest inside a country with the power of transforming it into a civil war and expanding at regional level, the triggering factors have yet to be discovered. From the period of the Qing Dinasty (1644-1911) in China to the Great Depression in the 1930s in the USA or the beginning of the second World War in France, these periods have two common points: civil unrest and food shortages.