Also known as the Ceaseless Fray
For at least a millennia, humanity toiled in their war against the Yill Maingess. Eventually the war ended and the Yill Maingess left the Milky Way Galaxy. Soon thereafter, the Stellar Decay began.
Duration: ~3500 CE to ~4500 CE (records are unclear)
Note: It is entirely possible (and true) that the war went on much longer. At the time, the government was in complete control of humanity. Records were intentionally obfuscated and history rewritten again and again, to keep morale high. This created an artificial gap in the historical record, not even stellar positioning can be used to reverse engineer the correct date, due to the extent of the alterations. This is a true testament to just how much control the government had over humanity at that time.
Even the Methuselans that existed at that time and their Academy Of Archivists, could not fully avoid the manipulation of history.
Technological stagnation:
In the early age of the war, technology accelerated rapidly. Spacecraft, weaponry and material sciences created marvels not sen again since this era. However, once the war reached it’s optimal stalemate point (which was a fallacy in and of itself), advancement was seen as an unnecessary complication.
Sustainment was the new goal for humanity. All new military technology was effectively banned by the government. In truth, we had reached a logical peak regardless. The new doctrine led to massively more efficient manufacturing and maintenance workflows. Allowing a factory to frontline system to feed the war effort indefinitely.
The war itself was never being won. We were simply feeding personnel and materiel into the maw of the still tolerant Yims. The alien combat ability was many magnitudes above our own, easily destroying everything we threw at them for the duration of the war.
The end of the war was a subject of much debate during the Second Golden Age. Some kind of plan was enacted by a group working outside the Sol Ascendency‘s direct purview. The exact nature of what that group did in unknown, but the results were clear.
The Yill Maingess ended up leaving the galaxy. The final battle resulted in the loss of almost all the colonies within about 15,000ly from the core, completely scrubbing the inner Norma Arm of human life.
The Sol Ascendency was forced to declare this a victory, even though it was counter to to their plans for endless war. Without the ongoing conflict, the government fragmented and humanity diversified. Over the next several centuries, the Second Golden Age began.