The Charon War was the largest conflict in human history with seven billion, out of the twenty eight billion humans alive being killed as a direct result of the conflict, and a further two billion dying indirectly due to the warfare. It was considered humanities first ever major galactic conflict that went beyoned more than one solar system, affecting hundreds and resulted in the overturning of the pre-existing balance of power in human space. The war lasted seven years and saw the deaths of 1/4 of humanities galactic population.
The Charon War began officially on the 20th July 2499 where on the Imperial Union planet of Crayt a nuclear warhead exploded in the planets capital killing its 3 million population. The planet had been playing hosts to holdouts of the H.U.M uprising and Imperial forces were steadily rounding up the last survivors when the Democratic Federation Special Forces were illegally dispatched to recover the survivors without the approval of the civilian government. While neither side is sure who dropped the missile, both ships were destroyed fighting each other and burned up in atmosphere and the Imperial Union issued a declaration of war in response.
However the causes of the war lye far deeper than a mere border skirmish. On one hand the the political views both factions held directly polarised both factions with the Imperial Union's desire for independence directly contrasting the Democratic Federation's call for unity. This fact underlined deeper themes of hatred in the outer colonies for the Earth based dominance and culture that permeated the inner colonies, with a stigma for culture and moral superiority long dominated human politics, with inner colonists being accused of believing they could do no wrong.
Moreover repeated movement of undesirable people within the inner colonies during the dark century and later refugee exodus's in the years following the fall of the mega corporations leg to border friction as refugee's tried to flee inner colony fighting to the outer colonies. Many of these interactions led to murder genocide and racial tension from the new and existing colonists. In addition the outer colonies, particularly the solar north who made up the original founders of the Imperial Union, felt used by the inner colonies as nothing more than factories, and as such were often referred to derogatorily as a back water. Moreover the technological gap between the two areas meant that the outer colonies were forced to pay heavy taxes to them to import the inner colonies more advanced technologies.
However the pension for violence and invasion from solar north nations, particularly those with more Imperial mind-sets, often led to violence on the border, if not outright invasion of inner colony worlds. In addition to the outer colonies divided and fractured nature they developed frequently extreme and isolationist viewpoints which made them very hostile and prone to violence, as well as accepting of unusual and extreme political views and ways of thinking, making it very hard for the Democratic Federation to deal with them. Their stubborn nature also made it near impossible for major political or social reforms to be made, with many of the political systems supporting unelected monarchies and/or heavily controlled elected political positions which prevented vast amounts of their populations from voting, thus perpetuating systems that frequently left the power in the hands of the few. Add this with isolationist tendencies the political climate in the outer colonies made it near impossible for rapprochement and reconciliation of past differences in the mid 2400's. While such isolationistic and extremist tendencies can be blamed on early inner colony actions and viewpoints of the outer colonists, the stubborn refusal to accept apologies meant that the war become assured by their inability to accept change.
However the outer colonists saw it as necessary to protect their way of life from governments that had no right to infringe on their political systems and had already been politely asked to not interfere with them. In fact this protection f their way of life was worked into a frenzy when the Democratic Federation lifted the bans on genetic modification and artificial intelligence to 'improve mankind.' Due to the colonial ideology of Zoh Zum or hard work and rising up from the bottom to the top, the potential people could be vetted by their genetics was just seen as another means of control by the richer inner colonists over the outer colonists. And beyond that was the general fear and stigma mankind had against A.I's for centuries after the Crash rebellion during the later 21st century.