The Roman Empire cracked under the pressure of mass migrations of well-armed peoples forced to move West by climatic shifts.
Perhaps there is a lesson there.
Otherwise I agree.
Sorry that this is the best citation I can find:
https://www.vox.com/the-big-idea/2017/10/30/16568716/six-way...
"Tree rings suggest that a megadrought in the middle of the fourth century might have made these nomads desperate for greener pastures. As they migrated West, they terrified the highly developed kingdoms, such as those of the Goths, that had long existed along Rome’s frontier. Partly because of this climate-caused upheaval, the Goths challenged Rome’s frontiers as never before. Rome’s Western territories ended up being carved up and reconfigured as Germanic kingdoms."
The Roman Climatic Optimum is the name of a warm period from roughly 250 BC to AD 400. It wouldn't be a stretch to hypothesize that the fading of that warm period may also have had effects on the steppes, leading to the migrations.
The Visigoths may have been fleeing the Huns, but why were the Huns on the move in the first place?
The Tang dynasty, like the Roman Empire, splintered in internal divisions and revolts, leaving behind a rump state that preserved elements of the previous structure, as well as a plethora of semi-"barbarian" successor states controlling portions of the former extents that had their own synergies of "Chinese" and non-Han properties, which one can squint at and see similar reflections of in the Germanic Roman successor states and the Eastern/Byzantine Empire. When Genghis rode into the gates of Zhongdu (Beijing), he had driven out the Jin, who were originally Jurchens from Manchuria, who had in turn displaced the Liao dynasty, who where originally Khitans from Mongolia, who had in their turn taken over when the Tang dynasty fell to pieces. Then his grandson completed the conquest and wiped out the Song dynasty that had remained in control of the southern portion of the old empire. Imagine Charlemagne being coronated Holy Roman Emperor in Rome, and then rather than his grandsons dividing his empire between them in civil war, one had proved powerful enough to maintain control, and then turned and captured Byzantium and the rest of the former Eastern Empire.