The success of grain agriculture in providing an abundant source of calories for the human diet led to large increases in human populations, to urbanization, social stratification, and then to the establishment of armies and the frequent occurrence of warfare among neighboring human societies. The advantage of grains as an easily transportable, concentrated form of food has also directly contributed to human aggressiveness by enabling mobile warfare. Invading armies have been fueled by grain. The biggest empires in the world were built by societies that were particularly successful at growing and controlling grains. Furthermore, much of the action in warfare has involved combatants trying to obtain grain to feed their own side and to prevent the other side, including civilians, from having any. However, history shows that the greatest death tolls due to starvation of civilian populations have been caused by domestic disputes, with some of the biggest disasters occurring during the 20th century in totalitarian regimes.