A civil war is a war between organized groups within a single nation state, or, less commonly, between two countries created from a formerly-united nation-state. The aim of one side may be to take control of the country or a region, to achieve independence for a region, or to change government policies. It is high-intensity conflict, often involving regular armed forces, that is sustained, organized and large-scale. Civil wars may result in large numbers of casualties and the consumption of significant resources.

Civil wars since the end of World War II have lasted on average just over four years, a dramatic rise from the one-and-a-half year average of the 1900-1944 period. While the rate of emergence of new civil wars has been relatively steady since the mid-1800s, the increasing length of those wars resulted in increasing numbers of wars ongoing at any one time. For example, there were no more than five civil wars underway simultaneously in the first half of the twentieth century, while over 20 concurrent civil wars were occurring at the end of the Cold War, before a significant decrease as conflicts strongly associated with the superpower rivalry came to an end. Since 1945, civil wars have resulted in the deaths of over 25 million people, as well as the forced displacement of millions more. Civil wars have further resulted in economic collapse; Burma (Myanmar), Uganda and Angola are examples of nations that were considered to have promising futures before being engulfed in civil wars.

Scholars investigating the cause of civil war are attracted by two opposing theories, greed versus grievance. Roughly stated: are conflicts caused by who people are, whether that be defined in terms of ethnicity, religion or other social affiliation, or do conflicts begin because it is in the economic best interests of individuals and groups to start them? Scholarly analysis supports the conclusion that economic and structural factors are more important than those of identity in predicting occurrences of civil war.