Almost every nation has minority groups, religious plurality, and ideological divisions, but not all plunge into civil war. Sociologists have long searched for what variables trigger civil wars. In the modern world most civil wars occur in nations that are poor, autocratic, and regionally divided. However, the United States was one of the wealthiest and most democratic countries in the world at the time of its bloody civil war. Some models to explain the occurrence of civil wars stress the importance of change and transition. According to one such line of reasoning, the American Civil War was caused by the growing economic power of the North relative to the South; the Lebanese Civil War by the upsetting of the delicate demographic balance by the increase in the Shi'ite population; the English Civil War by the growing power of the middle class and merchants at the expense of the aristocracy.

Competition for resources and wealth within a society is seen as a frequent cause for civil wars, however economic gain is rarely the justification espoused by the participants. Marxist historians stress economic and class factors arguing that civil wars are caused by imperialist rulers battling each other for greater power, and using tools such as nationalism and religion to delude people into joining them. Also, recent evidence proved that the violence observed in civil war can come from spurious reasons. Not only are the causes of civil wars widely studied and debated, but their persistence is also seen as an important issue. Many civil wars have proved especially intractable, dragging on for many decades. One contributing factor is that civil wars often become proxy wars for outside powers that fund their partisans and thus encourage further violence.

Research related to the democratic war theory have studied civil wars and democracy. Research shows that the most democratic and the most authoritarian states have few civil wars, and intermediate regimes the most. The probability for a civil war is also increased by political change, regardless whether toward greater democracy or greater autocracy. Intermediate regimes continue to be the most prone to civil war, regardless of the time since the political change. In the long run, since intermediate regimes are less stable than autocracies, which in turn are less stable than democracies, durable democracy is the most probable end-point of the process of democratization. The fall of Communism and the increase in the number of democratic states were accompanied by a sudden and dramatic decline in total warfare, interstate wars, ethnic wars, revolutionary wars, and the number of refugees and displaced persons.