[I originally posted this at the Total War Center forums.]
What the hell happened in the 3rd century? The Roman Empire, Han-dynasty China, and Kushan Empire of India all became divided states, while the Parthian Empire fell to the Sasanians of Persia.
It never really dawned on me to connect the fortunes of these four great empires of mid-Antiquity, but it seems like their fortunes were somehow connected during the course of the 3rd century AD. In East Asia, the Han Dynasty of China fell into a state of civil war and was divided into Three Kingdoms by 220 AD (reunited by Emperor Wu of the Jin Dynasty by 280). The Kushan Empire of South Asia fragmented into smaller semi-independent kingdoms after the reign of Vasudeva I, hard-pressed by Sasanian invasion and a century later more invasions by the Guptas and Hephthalites. Further west, the Parthian Empire experienced domestic turmoil and rebellion, culminating in the rise of the Sasanians who, under their leader Ardashir I, quickly captured Persia and the Mesopotamian heartlands (ending the rule of the last Parthian king of kings Artabanus V). After years of Germanic invasions, dynastic usurpations, and political assassinations, the Roman Emperor Aurelian ended the Crisis of the Third Century by 284 AD with the reunification of the fractured Roman realm (conquering both the Palmyran and Gallic Empires), an achievement cemented by Diocletian and his Dominate reforms.
So what the hell went wrong in this century? Why did four of the greatest powers on earth suddenly fall into a state of chaos and political division? Were there larger underlying factors at play? The Antonine Plague, which had ravaged both Rome and Han China during the 160s (depleting an estimated 10% of the population in Rome) had small reoccurences during the 3rd century, but would this natural catastrophe somehow explain away the bigger political movements? Was there a terrible shift in the global climate leading to agricultural disaster and famine, and hence civil unrest and turmoil? Is there any sort of connective tissue we can sew together between these vastly different and distant states spanning the whole of Eurasia during the 3rd century AD? And if so, who in the academic community has ever written about this sweeping topic? I don't remember Gibbon ever attempting a correlation between the fall of these empires (or the reunification of the Roman and Chinese ones at oddly close moments in time).
This is something I actually think about quite a bit, and would be happy to go into greater detail later today when I reach a computer, but I want to raise a couple points quickly here.
There are several what one might call "ecological" explanations, such as plague and climate change. I don't find these very convincing for a variety of reasons. An enormous amount of hay has been made out of the Antonine plague, for example, but it isn't really tightly correlated chronologically to much of anything--except of course the string of military problems the historians explicitly connected it to.
The individual events are perfectly explicable through internal factors--we do not need a pan-Eurasia crisis to explain the collapse of the Han or Roman systems. This doesn't mean we should ignore the temporal coincidence, but it is something to keep in mind.
It was a "collapse" only from certain perspectives, as it went along with a strengthening of "peripheral" systems. Rome's collapse, for example, saw the rise of a vigorous Palmyra and the increase in power of Germanic kings. The third century saw the collapse of the Han, but also the rise of SE Asian states, and the massive expansion of trade networks in the Bay of Bengal. The fall of the Kushans went along with the rise of the Gupta, and the fall of Parthia was also a regeneration of power in the Iranian region under the Sassanids.
Well, I can't give a concrete answer but I can give one theory from the book "Why The West Rules-For Now." In it, he talks about the seemingly coincidental collapse of both the Han and Roman empires as a result of social development getting too high. "Social Development" was the unit he used to measure how "advanced" and powerful a society was. Advancement created what he called a "paradox of development", think globalization creating vulnerable economies and anti-biotics creating more resistant bacteria. Anyways, both the Han and Roman's were getting higher on this scale than ever before. I will update this post with the specifics once I can actually look at the book.