The Delta Sector War began in 2228 and continued, irregularly, well into the 24th century. The T.T.F. ceased operations altogether in the Delta Sector, but no formal peace or armistice was ever signed.
On one side was the Terran Trans Solar federation. The other side was a loose alliance of numerous small colonies; pirates and smugglers; and the insurgent Cult of Sirius; teamed together with Nova Liberia.
By all accounts the war should have been a quick and overwhelming victory for the T.T.F It had more than one hundred times the number of times than its opponents, as well as many more bases and virtually unlimited supplies. In the eyes of the interstellar community the Delta Sector Alliance' (as later historians called Nova Liberia and its supporters) only had one main advantage to counter the might of the Federation Navy: its member nations were fighting for their survival.
Of course the situation was not as simple as pure survival, but it is true that necessity spurred the Alliance to use every asset available to it. Meanwhile, the Federation had to keep most of its navy occupied patrolling the solar system and other core systems against raiders, pirates, and the prevention of colonial rebellion. Therefore, the actual number of ships sent to fight in the Delta Sector was only about five times the number in the Alliance navies.
Early on, the Federation won many victories against the rebel states. It was able to destroy the navies of several colonies and occupy their worlds within the first year alone. But it couldn't effectively exploit these wins. The occupation forces suffered heavily by guerrilla insurgents. The partisans sabotaged repair facilities and docked ships. They attacked marine contingents on planets and in space stations, stole technical data, intercepted Federation Navy transmissions.
Meanwhile, things were going much worse for the T.T.F. state, the Sirian Republic. The Cult of Sirius steadily stepped up resistance against them, in the first few years of the war.
Though the Cult did not directly ally with other colonies in the region, it was careful to time its attacks on Sirian forces when the T.T.F Navy was occupying other colonies. The Cult of Sirius conducted sabotage on space assets, along with guerilla attacks on the surface of Sirius Prime. When their power was strongest they changed their name from the Cult of Siruts to the Kmet, a holy word in their esoteric scriptures.
The Kmet declared a holy war on the heathen government of Sirius, and in one swift, but well-planned stroke, overthrew the planetary government. Members of the Kmet struck simultaneously in space. Cult sleeper agents made up nearly a.third of the enlisted sailors on the only three capital ships in the Sirian navy. They mutinied precisely as news of them planetside coup reached the Sirian fleet. This secured the new power from counterattack from space as the T.T.F. had left few operational warships in system. Just eight years into the near century-long war rebel forces had managed to seize the T.T.F's main base of operations and eliminate its only ally in the region.
All this time the rebel Nova Liberian Navy continued its lighting attacks on Federation space docks and repair facilities. It also raided federation supply convoys. In 2237, a faction in one occupied colony, Pacifica, managed to hijack Federation Navy attack craft and fighters. The faction used these to destroy the T.T.F Navy's repair base in orbit around the planet. Other colonies suffered retribution for their rebel activities, but the T.T.F. could not maintain their garrisons on any of the planets after the 2240's.
The war dragged on for decades, with the two sides stalemated—the T.T.F. unable to permanently destroy opposition, and the allies unable to oust the invaders from their region. There were periods of peace, but they usually only lasted until the Federation Navy managed to bring in reinforcements. Only the threat from the Avarians stopped T.T.F. incursions.
Despite the destruction the war caused it was a great boon for the navies. It spurred the greatest development of space warfare in human history. Most of the ship classes currently in the T.T.F Navy were designed during this conflict. And early versions of the fighters and missiles used throughout human space were born at this time. Shields were designed by the T.S.F. to counter pulse torpedoes designed by the Nova Liberian Navy. They weren't as good as was hoped, but the technology developed by the end of the war allowed shields to stack up. Thus, it doubled or tripled the protection, and blunted most of the threat from head-on attacks. Meanwhile, Nova Liberia redefined the star bomber's role of a planetary assault craft to a very potent torpedo boat lethal to even the largest capital ships.
In some ways the war was a victory for Delta Sector separatists. By I merely surviving, Nova Liberia was able to establish its sovereignty in the region, and unite with its once despised neighbors to fight off a greater threat. Unfortunately most of the colonies suffered too much to be glad of the exchange. Their governments were shattered; their people decimated; and navies obliterated. Some, like Pacifica, managed to seize enough Federation Navy ships, bases, and supplies to rebuild defenses and infrastructure. For many others the devastation was too extreme for rebuilding. People deserted them for more stable star systems. Thousands of colonists were forcibly resettled by the T.T.F. into internment camps on already inhabited planets in the Federation core systems.
Only two nations could claim true victory. The Kmet managed to gain control of its own star system and with it the oldest human settlement outside the Solar System. By the end of the War the Kmet had built the largest navy in Delta Sector. The Nova Liberians lost most of their navy, but managed to seed colonies into more than a dozen star systems.
source: Terran & Avarian Fleet Book