The Great War, also referred to as the War of Human Unification, the War of Human Liberation, the Human-Alien War, and the Ten Years' War, was an interstellar conflict waged between 8 September 3265 to 1 March 3275 which consumed the breadth of the Milky Way galaxy. It ultimately came to involve every interstellar major nation, and pitted its three largest alliances against each other in a three-way conflict. It was the most widespread war in known history, and directly involved hundreds of trillions of humans and aliens on all three sides, in which each nation committed its entire economy and society to the total engagement of war. The results were massive civilian and military death tolls beyond numbers reported in any previous conflict, at a devastating scale which saw the extinction of at least three alien species, and the severe depopulation of dozens of other races. The war ended with the human-led Interstellar Treaty Organization achieving total victory over its opponents, and the development of the human allies into a singular human nation-state: the Central Galactic Union.
The conflict had been simmering for decades since the end of the Cruxite War in 3235. The strategy of a massive arms race combined with intensive colonialism in its borders by the Terran Empire and its allies was designed to make war with humanity an unwinnable scenario for both the Soviet Republic and the alien alliance known as the Free International Group. The war is generally considered to have begun with Soviet attacks on the Orion Empire's border stations on 8 September. The immediate next day, the ISTO member states declared war on the Soviets in defense of Orionese sovereignty. War was waged between the Soviets and the ISTO from 3265 to 3271, when the Soviet Republic collapsed. At the same time, negotiations between the FIG and the Soviets broke down and prevented an alliance from forming between the two blocs; when the ISTO also attacked the FIG, the Soviets and FIG were engaged in a war of their own against each other. The conflict became a three-way war from 3266 onward when the ISTO launched an invasion of the Republic of Cassiopeia. The ISTO was reorganized in November 3270, centralizing command and control of its armed forces.
The Soviets lost the critical Battle of Khoz in late 3267, leading to mass armed mutinies of their military forces and the eruption of civil war. Individual worlds seceded from the communist federation and secured autonomy by force of arms, slowing down the war effort against the ISTO and the FIG. Within two years, the Soviet government had all but collapsed and its forces had suffered significant losses in a war of attrition. The legitimate government sued for peace and unconditionally surrendered in April 3271. A remnant faction retreated into deep space and continued a guerilla war. The Soviet Republic was dissolved and its planets placed under the occupational authority of the Confederate armed forces. The Confederation was then able to focus its entire force on the FIG states. After a little over three more years of conflict, in which the Confederation occupied the majority of member states of the FIG and secured their surrenders, the human military forces conducted a massive orbital bombardment of the United Suns' capital world and compelled the surrender of its rump government in February 3275. The war was officially declared over at the signing of a peace treaty on 1 March, in which all of the opposing alien governments were dissolved and their territories officially subsumed into the Confederation, which became the Central Galactic Union on 20 April.
During the preceding three centuries, the Terran Empire had ascended to become the strongest interstellar human-ruled state, and had forged military and political alliances with numerous other spacefaring nations. This system of alliances began in the 2500s with the Interstellar Treaty Organization. The ISTO came to be a military, political, and economic alliance that promoted a united front of the human nations, particularly emphasizing the traditions of monarchy, parliamentary democracy, and industrial-military power as well as the common cultural, historical, and racial ties as being dominated and ruled by humanity. National Humanism developed from the 31st century into the predominant human-nationalist ideology, rising with the National Humanist Imperial Labor Party and its foreign offices. The Terran Empire, Empire of Orion, Empire of Sagittarius, Federation of Cygnus, Norman Empire, Kingdom of Crux, and Despotate of Perseus composed this alliance.