Military conflict centred on territory within a state, involving combatants from that state, over the political right to control that territory. Civil wars usually involve government forces, and the territorial aspect of the conflict means that civilian involvement and civilian deaths are usually high. In practice, it is sometimes difficult to differentiate between a civil war and terrorist activity, coup d'état, ethnic cleansing, or international war. Academic attempts to quantify the incidence of civil war, for instance through the Correlates of War studies, have emphasized the requirement that state violence should be sustained and reciprocated, with significant numbers of casualties (typically more than 1,000 deaths, with significant casualties on each side of the conflict). Nicholas Sambanis has identified the onset of 119 civil wars between 1945 and 1999.
Historical examples show the variety of causes and the range of situations in which civil wars can occur. The English Civil War (1642–6) can crudely be characterized as the battle between forces loyal to parliament and forces loyal to the monarch. It did, however, encompass many other issues, notably religious toleration and the relationship between Church and State, and other countries, notably Scotland, Wales, and Ireland. The American Civil War (1861–5), fought between the southern Confederate states and the federal Union forces, centred on the rights of states within the Union, the legitimacy of secession from the Union, and the issue of slavery. The Russian Revolution (1917) was accompanied by civil war, with the conflict lasting until 1923. The fall of the Russian Tsar, and the international instability following the end of the first World War, also led to civil war in Finland in 1917. The Spanish Civil War (1936–9) pitted left‐wing Republican forces against the right‐wing Nationalist army. The schisms exposed by civil wars tend to be deep, and are often entrenched in the partisan and institutional politics of a state for years after the conflict has ended.
Attempts to explain and analyse the outbreak of civil war have overlapped with the study of revolution and war. Studies have looked at the incentives which may trigger civil conflict, the structural factors common to conflict situations, and the factors which lead to civil wars being sustained. Influences on the incidence of civil wars have included the roles of elites and the relationship between the government and military forces; economic factors, such as the distribution of wealth and income and the control of natural resources; religious, cultural, and ethnic divisions in society; and international intervention. The prevalence of civil wars in contemporary politics shows that greater understanding of their causes does not easily translate into measures to prevent them occurring.