What is Civil War?

Legal Definition
A civil war, also known as an intrastate war in polemology, is a war between organized groups within the same state or country, or, less commonly, between two countries created from a formerly united 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. The term is a calque of the Latin bellum civile which was used to refer to the various civil wars of the Roman Republic in the 1st century BC.

A civil war is a 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. Most modern civil wars involve intervention by outside powers. According to Patrick M. Regan in his book Civil Wars and Foreign Powers (2000) about two thirds of the 138 intrastate conflicts between the end of World War II and 2000 saw international intervention, with the United States intervening in 35 of these conflicts.

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-19th century, the increasing length of those wars has 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 20th century while there were over 20 concurrent civil wars close to the end of the Cold War. 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; Somalia, Burma (Myanmar), Uganda and Angola are examples of nations that were considered to have had promising futures before being engulfed in civil wars.
-- Wikipedia
Legal Definition
An internecine war. A war carried on between opposing masses of citizens of the same country or nation. Before the declaration of independence, the war between Great Britain and the United Colonies was a civil war; but instantly on that event the war changed its nature, and became a public war between independent governments. Hubbard v. Exp. Co.., io R. I. 244; Brown v. Hiatt, 4 Fed. Cas. 387; Prize Cases, 2 Black, 667, 17 L. Ed. 459; Central R. & B. Co. v. Ward, 37 Ga. 515.
-- Black's Law Dictionary
Legal Definition
Declared armed conflict between a government and another group from the same country.