A civil war, also known as intrastate war, is a war between organized groups within the same state or country. It is a high-intensity conflict that often involves regular armed forces. One of the reasons for the lack of consensus in the study of civil war is disagreement over what exactly civil war means. Theoretically, civil war overlaps with other categories of armed conflict, particularly revolution, political violence, ethnic conflict, and terrorism. Civil wars since the end of World War II have lasted for over four years on average, a considerable 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-19th century, the increasing length of those wars has resulted in increasing numbers of wars ongoing at any one time. Since 1945, civil wars have resulted in the deaths of over 25 million people, as well as the forced displacement of millions more, along with economic collapse. According to scholars of civil war research, the causes of civil war include economic motivations or greed, and political or social grievances. Greed-based explanations focus on individuals’ desire to maximize their profits, while grievance-based explanations center on conflict as a response to socioeconomic or political injustice. A third concept, opportunity-based explanations, talks about factors that make it easier to engage in violent mobilization.