I have long been perplexed by the Tibetan Empire, which existed from 618 to roughly 842 CE. The Tibetan Plateau is a sparsely populated land with a challenging physical environment. Not surprisingly, for most of recorded history it has played a minor geopolitical role. Yet for more than 200 years, the Tibetan Empire was something of a superpower, fully competitive with the Tang Empire of China, the Muslim caliphates of the Middle East, and other great Eurasian states. As the maps posted below indicate, the Tibetan Empire’s territorial sway extended, at various time, well beyond the plateau itself. At its height it covered the Tarim Basin of northwestern China (Xinjiang), the Yunnan Plateau, and more. 
The most extensive depiction of the Tibetan Empire that I have found was published in a recent Science Direct article (Hou et al., 2023). On this map, posted below, the Tibetan Empire in 820 CE is shown (with a dashed yellow line) as having included most of what is today Afghanistan, Uzbekistan, Tajikistan, and Kyrgyzstan, as well as much of Bangladesh and lowland north India. Most maps of the empire show it as having exercised only indirect power over a much smaller part of northern South Asia. Animated maps, moreover, indicate that its power over this area was brief. The map posted in Hou et al. thus seems to be exaggerated, as is so often the case in the mapping of empires. 
Regardless of cartographic hyperbole, the article in question does offer an intriguing and largely convincing explanation for the rise and fall of the Tibetan Empire. As the authors’ investigation shows, between 600 and 800 CE the Tibetan Plateau experienced both relatively warm and wet conditions, allowing both agriculture and pastoralism to thrive. Such circumstances, in turn, provided the economic basis for a strong and expansionistic state. Importantly, this period was the only time in the past several thousand years that saw both warmth and “exceptional” humidity on the lofty plateau. Significantly, the collapse of the Tibetan Empire coincided with a return to drier conditions. As the authors write:
A closer examination at the precipitation records revealed that periods when Tibetans actively invaded Tang territory mostly coincided with humid periods, while conversely, Tang invasions into Tibetan territory occurred during relatively dry periods on the TP [Tibetan Plateau].
The peak of the drought at approximately 840 AD coincided with the collapse of the Tibetan Empire. Cold and dry events would be expected to lead to a decline in crop production, breaking the balance between population and resource supply.
Other major episodes of imperial expansion in Central Eurasia also seem to have been linked to favorable climatic conditions. A 2014 article, for example, found something similar in regard to the Mongol Empire. As the authors summarized their findings:
A 1,112-y tree-ring record of moisture shows that in opposition to conventional wisdom, the climate during the rise of the 13th-century Mongol Empire was a period of persistent moisture, unprecedented in the last 1,000 y. This 15-y episode of persistent moisture likely led to a period of high grassland productivity, contributing fuel to the Mongol Empire.
A century ago, leading geographers argued that climate and other aspects of the physical environment determine the course of human history. Such overweening determinism weakened the discipline, contributing to its subsequent marginalization. Today, however, historians and physical scientists are reviving the study of the influence of climate – and of climate change – on the course of history, but are doing so in more modest, rigorous, and productive manner.
Environmental determinism reached its height in the early twentieth century in the works of Yale geographer Ellsworth Huntington. In 1907, Huntington argued in The Pulse of Asia that a long period of intensifying droughts beginning several thousand years ago forced Central Asian pastoral nomads out of their homelands, impelling them to conquer sedentary states located in more climatically favorable areas. As he put it (page 393-394):
If the rainfall fell from 20 inches to 10, the number of sheep would decrease from 60 to one. Manifestly, if such a change took place in the course of a few hundred years, most of the inhabitants would be obliged to migrate. As the nomads pressed outward from the dryer central regions of Asia, we can imagine how they were obliged to fight with the neighboring tribes whom they tried to dispossess. The older inhabitants and the newcomers could not all live together; new migrations would be a necessity; and the confusion would spread in every direction. Meanwhile, Europe, after this long period of blighting cold, was becoming warm and habitable, and the migrants pressed into it, horde after horde. No one tribe could stay long in its chosen abiding-place, for new bands of restless nomads pressed upon it. Rome fell before the wanderers. Nothing could stay their progress until the turn of the tide.
Per chance, though this is only vague conjecture, the legends of King Arthur and his Knights bear a hint of what might have occurred all over central Europe if it had not been for the influx of Barbarians. England, in its remote corner of Europe, far from the dry plains of Asia, responded at first to the influence of improved climatic conditions, until it, too, was reached by the migrating hordes of invaders. Meanwhile, in the most densely populated part of Asia, another movement of nations had begun, presumably because of the distress due to rapid desiccation. The Arab migrants carried with them the fanatical faith of Mohammed, and were by it inspired to remoter conquest.
Such conclusions are directly opposite of those now being advanced by scientifically informed historical studies. One could not, of course, expect Ellsworth Huntington to have had a modern conception of environmental and historical processes and conditions in 1907. Still, to have argued, if only as “vague conjecture,” that the mythical glories of Camelot might have been replicated over central Europe in the early Middle Ages, had it not been for the drought-driven barbarian invasions, is more than a stretch. Unfortunately, the excesses of Huntington’s determinism led geographers to largely abandon the idea that the physical environment has any significant influence on human history, much to their detriment. Today, scientific studies of the changing physical environment are clarifying our conception of the human past are helping to solve historical mysteries.