The war began before anyone fighting it today was born. The exact start of the conflict remains disputed, but most historians would agree the secession of planets declaring independence from the United Empyreal Federation was the catalyst which set the greater conflict in motion.
Following a long period of economic reforms, rollbacks in social safety nets, and consolidation of power into fewer and fewer privileged hands, resistance grew among the laboring people on various planets under Federation control as those elected to represent the people failed to do so. This trend continued, although it could not continue forever. Incremental changes over time pushed the working people to their breaking point, and they began to push back, demanding a greater say in the systems controlling every part of their lives. Too many people had been held down for too long, deprived of the basic needs they deserved as human beings and left with no legal recourse by those in power as the electoral system continued to produce the same unwanted result of preserving the inequities of the status quo.
Thus, the course was set for revolution.