Nobody is quite sure who started the fighting, but it is known that the fighting spread quickly. Conventional warfare lasted for about a day before one faction used one of the strategic weapons that had been left behind by the Andromedans. As quickly as the fighting had spread the use of these weapons spread. Worlds were destroyed, entire star systems were annihilated. Most of the surviving worlds did so because the weapon used was mishandled, or it did not work the way it was supposed to, like in the case of Titus IV where the chemical weapon deployed there permanently mutated the surviving population instead of killing all of them. The worlds that did survive, such as Earth, were decimated. Their entire infrastructure had collapsed leaving entire planets without food, water, electricity of heat. The death toll on Earth was in the billions, though the population was sill over 50 billion. The war was over in a matter of days. The war had started on the eve of first contact with the rest of the galaxy. The other races were stunned by what humanity had inflicted upon itself and knew that humanity still possessed enough of these weapons to cause serious harm to any of the other races, even the Nan'Tra'Ki the one race that lost the fewest worlds to the Andromedans. The UNE came out on top of that war simply because they possessed Earth and Artech, the two most heavily populated and most industrialized worlds in Terran space. They dissolved the other surviving factions and absorbed what little militaries they had.
When the Great War began the human race had very little technology that they could call their own. They also could not replicate a lot of the Andromedan technology. Gravity generators and stellar cold fusion reactors were about the extent of what they could replicate. Other than that they were blasted back into the industrial age. Very few ships survived the war, and those that did were too low tech to be considered threats. Combined the UNE had a total of 105 ships of various size to defend its territory and enforce the peace. Of those 105 ships 32 belonged to the religious group calling themselves the Federation Commonwealth. They were located at the far eastern fringe of Terran space near the galactic rim next to the area where the Andromedans had retreated to.
The Federation Commonwealth blamed the war on high technology. They forsook any technology deemed to be at fault including the inertial compensators which made their ships unbelievably slow and their missiles increasingly unreliable due to short ranges. However they did possess enough ships to overpower the local UNE garrisons and they had been largely untouched by the war due to their low technology which made them a low priority target. The first crusade against the UNE began in 3273 and they either over powered local UNE ships or had them join the crusade. They over ran 15 star systems before the UNE could amass a force of 20 ships to oppose the now 42 ships of the Federation. The UNE was disparagingly outnumbered by the Federation ships and the technologies they did possess was not necessarily military technology. After the battle of Selenia III the federation took control of a freshly built drive yard were ships were being built to fight the federation. These ships were modified and turned against the UNE. The only operational ship yard close to the front lines was now pumping out ships for the Federation increasing their numbers advantage.
The UNE caught a break. Scientists and Engineers managed to reverse engineer a rare and priceless example of an Andromedan tachyon cannon. The process destroyed the only example of this advanced technology but the result was the photon cannon. A single cruiser designed by the commonwealth, the earliest form of the Provincial cruiser, was armed with several different examples of this weapon and sent into battle without a shakedown cruise. The ship performed well enough, it had several systems fail from routine malfunctions that would have been sorted out in a shakedown cruise. One of the ships primary turrets even failed, but the weapons range advantage and penetration capabilities far outmatched the weapons the Federation could field. A dozen ships were destroyed before the cruiser was forced to retreat. The Federation advance was temporarily halted only 12 jumps from Earth. More ships with the photon cannon were rushed into service. These ships turned the tide even though they were haphazardly assembled and rushed into battle. The Federation still managed to make some progress though at greater and greater cost. They captured several systems but at Holdfast and God's Reach the UNE made its stand with an all photon cannon armed fleet including a battleship armed with heavy photon cannons. The Federations meagre plasma cannons could not penetrate the hull of this massive battleship and it dominated the battlefield. It was transferred between the two systems as needed and the Federation lost over 40 ships depleting their military. The war was quickly brought to an end. The UNE was tired of war and did not desire to conquer a people that did not wish to be a part of the UNE. The Federation set up their capital on Selenia III where they had their greatest victory against the UNE. Over the course of 300 years the Federation had launched Crusade after Crusade against the UNE, but the UNE militaries had advanced beyond the capabilities of the Federation and they were not a threat. The Federation Rebellion was over.