Thoughts on Dark History in Turn A Gundam: The Dark History became so bad not because humanity kept fighting themselves, but when the universes started to collide everyone got confused and scared and fighting started back up. With the various techs of different versions of Earth, advancements happened rapidly, thus the alien origin of the tech in Turn A and the pilot losing their mind and attempting to destroy Earth.
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This is not without standing as we see that UC lasted for over 10k years before it came to an end but most of the fighting was in the first 2k years with almost no even "new" records in the Dark History archive until Turn A itself destroyed earth.
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Turn A went down in history as the protector of Earth though meaning before its turn, the pilot was an Amuro type, possibly a new Uso who completely collapses into despair after it comes to light that the aliens are all variants of Earth, showing that humanity had never once found a path to peace across any iteration.
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Oh additional thought: The Turn X was created to try to stop the Turn A when the pilot went mad, but failed to do so. Its why its the same tech, but different build and has less information left on it. All we know is it fought the Turn A and lost in the Dark History.
Please note that while there is strong evidence for some of this, it's pure interpretation based on the data presented in the series.
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Mg Avaris Clari