These are the factors that resulted in the outbreak of war:
1. Cultural divide between Earth and various colonies, thanks to advances in genetic engineering.
2. Failed negotiations between Earth and autonomists.
3. Growing secessionist sentiment in light of human rights violations by the UN's military.
4. Earth trying to directly control the supply of phosphorus, which is necessary for fertiliser and life support systems on human colonies (mainly to keep people on Earth from starving, since Earth had run into peak phosphorus).