The Sagittarian Civil War, called also the Sagittarian Revolution and the Five Hundred Year's War, was a military and political conflict waged between the Empire of Sagittarius and the Sagittarian Free State between the 2770s and the 3270s. For much of its duration, it was a low-intensity conflict reminiscent of a "cold war", with intermittent periods of direct conflict. The latter stages of the war were enveloped into the Great War.

The war originated from the seizure of several worlds by the Free Sagittarian Army, a rebel coalition that opposed the transformation of the Sagittarian Republic into a monarchy in 2774. The rebellion grew in size, and the power of the communist faction of the rebels grew as well, until a relatively stable front had developed by 2800. The rebel-held worlds and space stations in the spinward and trans-core region of the Sagittarius arm of the Milky Way galaxy became part of the Sagittarian Free State, which claimed to be the legitimate government of the Sagittarian territories and the successor state to the Sagittarian Republic. For almost the entirety of its existence, the Free State was heavily supported by the Soviet Republic and was effectively a satellite state of the communist bloc known as the Communist Interstellar, and came to enact Soviet-influenced policies of land collectivisation, alien emancipation and empowerment, and the strict control of political process by a coalition of communist parties. By contrast, the Empire of Sagittarius remained an independent power and only later came to be a strong ally of the Terran Empire in the 3000s.

Roughly 600 billion soldiers and two trillion civilians died in the course of the war. For most years, casualties on both sides averaged two million dead and wounded due to periodic military actions. However, some periods of intense conflict saw tremendous battles of attrition which killed tens of millions. The conflict deflated the manpower of both states, and permanently damaged the industrial capacity of the Empire of Sagittarius, which had lost its original "industrial belt" of worlds to the initial rebellion. It is now suspected by historians that this decrease in industrial power led to the decline of the Sagittarian state as the "galactic face of humanity" and its eventual subservience of the Terran Empire as a member state of the Interstellar Treaty Organisation.

Community content is available under CC-BY-SA unless otherwise noted.