Mer-Sam'ti War

The Mer-Sam'ti War (Ur lit. "Greatness Ending War"), sometimes called the 5 Years' War was an interstellar was between the Mirdesh Federation, Surben Republic and Ur-Han. It lasted from 2317-2322. This war ended Ur-Han overwhelming dominance for a time and permanently ended the Ur-Han Golden Age. 
In 2300, the Ur-Han king, Marrum the Third died. He is considered to be the last ruler of the Golden Age. His son became king. He was assassinated in 2302, when he was only 5 years old. Having no male siblings, the crown went to his cousin, Orapt the First. Orapt hadn't been prepared to rule, and as such began his reign as a king who delegated nearly all tasks to his court. He spent most of his time partying. This was not popular with the Ur-Han public, and some anti-monarchist sentiments began to appear. In 2308, the Duke of the Surben Duchy (territories integrated into the Ur-Han at the end of the Second Telom War) was involved in a massive corruption scandal. The company he owned, Hlorm-Fa Sel, was openly utilizing Surben and Telom employees as slaves in everything but name. This was made possible thanks to his influence at the royal court and gigantic bribes the company was giving out to officials. The Duke was the brother-in-law of the king, Orapt, and he was exempted of all charges. The Surben and Telom slaves continued working for the company. In 2310, Orapt proclaimed that slavery was now legal for non-Ur citizens of the kingdom, without consulting the Mastatolur, the Ur-Han senate. Whilst the Ur-Han are a feudal monarchy, the Mastatolur is made up of the greatest sages from the kingdom, and disregarding them completely was seen as an insult to Ur-Han traditions. Except for a small minority, the vast majority of the Ur were opposed to slavery. Mass protests erupted across most Ur-Han worlds. Worlds with majority alien populations entered open revolt and demanded the abolishment of the monarchy. By 2316, the Ur-Han were definetly out of their golden age and had entered a civil war.
In 2317, the Mirdesh and Surben declared war on the Ur-Han. They were rapidly able to recapture the worlds they had lost in the previous war. The Ur-Han armed forces were nearly exclusively focused on suppressing all republican uprisings. By 2322 the Ur-Han had capitulated. At this time, the Ur-Han had made first contact with the Terran Federation and had started the First Contact War. Unbeknownst to them, this nation would bring their demise over three centuries later.
The Ur-Han were forced back to their pre-Second Telom War territory. Their population was halved in the process. The Surben Republic decided to annex the area of the previous Telom Nation as an autonomous province instead of making them independent. However, they maintained a disproportionally large navy, as their naval forces had taken nearly no military losses during the conflict.
Many modern scholars, Surben, Ur-Han or Mirdesh think the war could've gone differently if the Ur-Han king was focused more on fighting the Mirdesh and Surben than the republicans. Many Ur-Han view Orapt with spite, as they believe he knew he would be able to retain his extravagant lifestyle, even if the nation lost colossal amounts of territory but not if the monarchy was abolished.