The Finnish Civil War (Finnish: Suomen sisällissota, kansalaissota; Swedish: Finska inbördeskriget) was a part of the national, political and social turmoil caused by World War I (1914–1918) in Europe. The Civil War concerned control and leadership of the Grand Duchy of Finland as it achieved independence from Russia after the October Revolution in Petrograd. The war was fought from 27 January to 15 May 1918 between the forces of the Social Democrats led by the People's Deputation of Finland, commonly called the "Reds" (Finnish: punaiset, Swedish: röda), and the forces of the non-socialist, conservative-led Senate, commonly called the "Whites" (Finnish: valkoiset, Swedish: vita). The Reds—usually Finnish-speaking workers—were supported by the Russian Soviet Republic; they were based in the industrial cities in the south. The Whites—dominated by farmers and middle- and upper-class Swedish speakers—received military assistance from the German Empire. The Whites won the war, in which about 37,000 people died out of a population of 3 million.

A civil war is a war between organized groups within the same nation state or republic, 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. 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.

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 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 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.