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A civil war is any war that occurs within a state, rather than among states. Most modern civil wars start due to nationalism, either as break-away movements by nationalities within a state or to "right wrongs" that have been committed against a particular nationality. Sometimes this takes the form of a "holy war," as well, as different religious factions line up against each other.
Due to the extremely high feelings created by a civil war, they tend to be the bloodiest and most intractable wars possible. Examples of this include the civil wars in Rwanda and Yugoslavia. Nowadays most other nations are loath to interfere in a civil war for this reason. However, civil wars often have effects beyond the border of the state experiencing it, as refugees flee to bordering states or combatants try to use neutral states as a hiding ground.
There have been many cases where a civil war has changed history for the entire world. Prominent examples include the Russian Civil War (the birth of the Soviet Union), the Spanish Civil War (when Hitler and Mussolini backed Francisco Franco), the American Civil War (when the US government became more centralized after defeating secession), and the Chinese Civil War (when the Communist Party came to power and the Peoples' Republic was established).