A civil war is a war in which the opposing parties are members of the same race, culture, society or nationality.
- American Civil War - A war fought between the United States of America and the Confederate States of America - which was comprised of former states that had broken away from the Union. The war was fought from 1861 to 1865, and was a key event in the elimination of slavery and repression on Earth.
- In 2246 civil war broke out between the Vestians. Rebels from Vestos II threatened to have the war spill out of Vestian space so Starfleet dispatched the USS Aldrin to patrol the Vestios sector to make sure hostilities were contained within Vestian space and did not disrupt normal shipping. The war was quelled in that year with the rebels surrendering to the Vestian authorities. (TOS novel: Burning Dreams)
- The planet Wyath suffered a civil war following the exile of Kommandant Ghud who had been ruling the planet. (TNG comic: "Light of the Day")
- Klingon Civil War - A brief war fought between two factions in the Klingon Empire in late 2366 and early 2367. Factions loyal to Gowron were able to defeat the Romulan backed Duras sisters after the Federation exposed Romulan involvement in the war. This helped secure Gowron's position as Chancellor, and for a few more years preserved an alliance between the United Federation of Planets and the Klingon Empire until Gowron withdrew from the alliance when the Federation condemned a Klingon invasion of Cardassia. (TNG episode: "Redemption" and DS9 episode: "The Way of the Warrior")