The Battle of Bilbringi was what most historians agree on as the final nail in the New Republic's coffin. In one day, then-Grand Admiral Thrawn had defeated and captured the New Republic's most talented and experienced fleet, and captured Admiral Ackbar with it. Some smaller ships refused to surrender and were destroyed, but for the most part the surrender went smoothly.
Supreme Commander Thrawn's victory brought hundreds of systems back into the folds of the empire. From the Outer Rim to the Core the Imperial flag was raised. Over the course of the next few weeks more and more systems joined the Empire once more.
The New Republic was divided by infighting between three main factions, further crippling its ability to fight back against the unstoppable forces of the Galactic Empire. General Bel Iblis argued that the New Republic should keep fighting until the end, Borsk Fay'lya advocated for returning to the days of the Rebellion, and Mon Mothma pushed for negotiations with the Galactic Empire.
Mothma did not initially want to, but after a conversation between her and Ackbar over the holocomm, she realised that the New Republic was doomed. The Admiral was blunt and advised surrender. Ackbar did not use any of his code words to signify anything else; he genuinely had lost faith in their effort. Mothma believed that through negotiations the galaxy could be spared the troubles of the old Empire.
Mon Mothma had the support of Leia Organa-Solo, a hero of the New Republic, which brought significant support to her side. Luke Skywalker, the Last Jedi Master, fled to the unknown regions with Mara Jade, only speaking with Han and Leia beforehand. No one knows why he left.
As the peace faction of the fracturing New Republic gained a significant majority, Iblis and Fey'lya put their difference asides and splintered off from the New Republic, taking a portion of the fleet with them.
The then-Grand Admiral Thrawn finally arrived in the Coruscant System shortly after and took it without firing a single shot.
Negotiations immediately took place, resulting in the Treaty of Coruscant. The treaty dissolved the New Republic entirely. The Second Galactic Empire was proclaimed, with Thrawn as it is Supreme Commander. A peaceful transfer of power was to ensue. High Command and the senators were not executed, however. Seeking to build bridges, not destroy them, the Supreme Commander offered High Command positions in the new government in both the bureaucracy and the military. There was much transitioning to come, but this was an enormous step towards Galactic progress.
This day was the end of the New Republic, but the beginning of something better.
Glory to the Empire.
(Except from Historia Galactica, a history textbook favored in the Mid Rim, Outer Rim and the Unknown Regions)
