From Gineipaedia, the Legend of Galactic Heroes wiki
By the early 27th century CE, the natural resources of the planet Earth had been depleted. In order to survive, the United Earth Government began to adopt an increasingly colonialist attitude toward its colony worlds, using its military force to stifle their economic and industrial growth. Wholly dependent upon imports and taxes from the colony worlds to survive, Earth needed the colonies to remain impoverished agrarian states, with export-reliant economies dependent on trade with Earth.
These policies were very detrimental to the colonial economies, and because the colonies were only afforded partial representation in the Humanity Congress, it was impossible for them to effect any real change to this policy of exploitation. The people of the colonies became increasingly resentful of Earth's heavy-handed policies, and in 2680 CE the Humanity Congress held open hearings on the inequality of the galactic trade policy. The colony worlds were angry, claiming to be "starving" due to overtaxation and the forced export of agricultural goods. These hearings accomplished little more than to increase the ire between Earth and her colonies: one UEG congressmen even went so far as to compare the attitude of the colony citizens to that of ungrateful slaves. The colonial delegates renounced the Earthborn plutocracy as being little more than pirates, pillaging the colony worlds for their own material gain.
In 2682 CE, the colony worlds formed a loosely-knit coalition called the Anti-Earth Movement, and presented the Humanity Congress with a trio of demands. The 2682 Ultimatums were also ineffectual, but caused the UEG to see the looming threat of war. The UEG launched The Sirius Threat propaganda campaign in an attempt to diminish hostilities, but the campaign ended up unifying the opposition behind Sirius. In 2685 CE, the Anti-Earth Movement declared its independence from the United Earth Government, and established the Sirian Revolutionary Congress.
The Occupation of Rondolina
In 2685 CE, the United Earth Government's Space Force sent a fleet to Rondolina to participate in a joint military exercise, ostensibly to further the cause of cooperation between Earth and her colonies. Under the pretext of providing artillery fire for the exercise, the Space Force launched a pre-emptive attack on the United Colonial Army before it even launched. With its military destroyed, the Sirius Revolutionary Congress fled Rondolina, and the UEG forces occupied the planet. This marked the beginning of the Earth–Sirius War.
The occupation of Rondolina's capital city by the Space Force was unconscionably brutal. Undisciplined and unscrupulous, the UEG soldiers beat civilians in the streets and looted the capital city. Civilian casualties numbered 600,000, but the UEG over-reported that number to Earth, saying that there were 1,500,000 casualties. In order to match the estimate to the reality, the Space Force slaughtered civilians and dismembered corpses. The occupying UEG forces also under-reported the number of their own casualties, so that corrupt officers could embezzle the salaries of fallen soldiers. These facts would eventually be exposed by a war-correspondent from Earth, but the guilty soldiers were all exonerated in the 2690 Military Trials.
The occupation quickly worsened, culminating in the notorious Bloody Night massacre in Laglane City. The brutality of the United Earth Government incensed the colony worlds and strengthened their resolve. (LOGH: 'To Earth')
The Battles for Vega
In 2691, the four leaders of the Anti-Earth Movement, Carle Palmgren, Winslow Kennes Townshent, Joliot Frankul and Chao Yuiling met on the colony world Prosperina, in the Proxima Starzone. There, they reorganized the Anti-Earth Movement into the Black Fleet, a military force drawn from all of the colony worlds wholly dedicated to the destruction of the United Earth Government.
After liberating Rondolina some time prior to 2703, the Black Fleet fought two decisive battles against the United Earth Government in the Vega Starzone. The Second Battle of Vega was an overwhelming victory for the Black Fleet, and shifted the tides of war in their favor. After that victory, the Black Fleet was quickly able to push the UEG forces back to the Sol Starzone.
The Fall of Earth
In 2704 CE, the Black Fleet, under the command of Joliot Frankul, penetrated the outer Solar System and quickly captured the the outlying space. The UEG's Space Force was defeated quickly and the Black Fleet pressed on to Earth. Once in orbit of Earth, the Black Fleet began to bombard the planet from orbit, killing billions of innocent civilians. With the United Earth Government annihilated to the man, the Earth–Sirius War drew to a bloody close. (LOGH: 'To Earth')
the Earth–Sirius War is remembered as one of the most violent conflicts in human history.