There have been four major civilization collapses on a global scale
The late bronze age collapse
The Indus Valley collapse (the reason why India is STILL a shithole)
The Mesoamerican decline
And the fall of the Roman Empire
Each of these collapses were caused by new groups of invaders trying to settle on already established lands, systems and governments.
The Bronze Age was very advanced even if we're talking more than 4000 years ago, they had global trade from England to China, coinage, literature, writing, soyciety, jobs, states, it was the Roman Empire but without the Empire and more like a bunch of army states all depending on each other for necessary trade. Then the Sea People came, people of the Islands, probably Mycenaean or Sardinians/Nuragics, they used boats to get across the Mediterranean and the first thing they did was set fire to all the cities they could find until there was nothing left. These then mixed with the present population and that's why the Middle East looks exactly like some parts of Southern Europe. Ironically these same Sea People would later establish the first Hellenic kingdoms, and get wiped out not much time later by the eruption of the Santorini, plunging the whole region into a state of ignorance, small time agriculture, villages and wilderness
The Indus Valley and the Mesoamerican ones are pretty similar, climoecological changes spelt the doom of the basics of these civilizations, which were later replaced by simpler ways of living introduced by moving populations. In the case of India, they had working sewage systems, astronomy, mathematical knowledge, cloth manufacturing and agriculture expertise. These were all lost when the typical p*jeet started moving in after the decline caused by various droughts and changing river formations, morphing the sub-continent into a low yield farm for centuries, effectively stagnating any sort of progress the soon to be higher castes managed to accomplish
Mesoamericans as was said are the same, changing climate conditions, social decline that hinted at degrading traditions, and groups from other regions moving in and causing chaos, which actually continued until the Spaniards arrived en masse and killed them all
The Roman Empire is probably the more known of these four
A functioning pre-steam society, with one form of governance, one culture, a functioning army, strong social bases, laws, infrastructure, global trade. Which came to an end when
Post too long. Click here to view the full text.