I have been researching Buddhist history for several years, and will summarize here what I believe to be the most likely narrative regarding the social, political, and religious history that led to the so-called ‘Dark Ages’ of Tibet. All of the information here comes from academic sources; I will not resort to footnotes at this time but will publish this material later in a more complete form.
As this site looks at the nature of Tibetan religion and society in the ~150 years after Langdarma, it seems worthwhile to look at the influences that led to the dissolution of the vast and powerful empire and the nascent Buddhism that was a part of it. There is a lot less information about what the Dark Ages and Tibetan Buddhism during that period were actually like, though some details are known, more are appearing, and others can be inferred.
In its location between the great states of India, China, Khotan, Oddiyana, and others where Buddhism flourished or was the state religion, Tibet was certainly aware of Buddhism for hundreds of years before the mid-eighth century CE events described in the well-known story of Trisong Detsen, Santarakshita, and Padmasambhava. Tibet at this time was in the process of becoming a powerful empire both economically and militarily. Central authority in the form of the king was expanding, though he still relied upon powerful clans that exercised great power in their own fiefs and whose allegiance supported his rule. The king needed these local lords for tax collection, and to provide men, horses, and provisions for the growing army. There were constant rivalries between clans, jockeying for position and influence.
In this atmosphere, the religion of the great powers of Asia possessed many features that seemed useful to an emperor wishing to create a more unified country, a more unified court, and simultaneously project a more civilized image to allies and enemies. Today most Westerners think of Buddhism’s benefits in terms of peace of mind, harmonious relations, compassion, and morality, but in the world of Trisong Detsen, Buddhism had many more important features.
First, Buddhism took root in Tibet together with a written language, including an alphabet linking it to India. A common written language would be critical for knitting together and managing an empire. Second, understanding and practicing Buddhism required education and training, something limited to a wealthy elite able to focus on study rather than livelihood. Buddhism at this time was almost exclusively a religion of the nobles, promoted and patronized by the king, and participation in a common training was seen as a unifying influence on the young men of the clans. Third, Buddhism at the time was well known to be both the Sutras and the Tantras. This was the age of the Mahasiddhas in India and Tantra was at the peak of its influence there. All forms of spiritual practice in the age were seen to be highly practical technologies for maintaining good relations with friendly gods and other entities, and warding off the negative effects of malevolent ones; the Tantras were especially respected in this way, even feared for their power. Translation and practice of the Tantras was restricted to circumstances tightly controlled by Trisong Detsen and his administration. Debate continues as to whether there was an organized, codified Bön religion in Tibet at the time of Buddhism’s rise, but there was undoubtedly a widespread belief in the power of gods and spirits, and a class of practitioner employed to perform ceremonies or otherwise intercede to secure good harvests, heal the physically or mentally ill, guide the spirits of the dead, divine outcomes and generally protect society, especially the noble clans.
The common people were taxed to build and support the new monasteries, as they had always been taxed by the nobles, and by the king to support the army of empire. As the monasteries gained respect and influence, land was also granted them by the king or clans. Monks became some of the most highly educated men in the country and were called to service as government administrators and even cabinet ministers. Thus the status and political power of the monastic institutions grew—starting to take a form not unlike that of the clan nobles. In the period before the dissolution of the empire, the monasteries’ power reached its peak in the reign of king Ralpachen, who was apparently a fervent advocate of Buddhism in general, both Sutric and Tantric.
Throughout the rise of Buddhist power and influence over the approximately 100 years since Trisong Detsen, there had always been those clans whose local bases of power or respect owed more to traditional beliefs or alliances than those Buddhism seemed to offer. When a series of bad harvests strained resources at around the time of Ralpachen, much of Asia destabilized. The Uyghur state to the north collapsed under pressure from the Kyrgyz in year 840, and many displaced people fled to Tibet. The great Tang dynasty in China stumbled around this time and fell shortly thereafter.
In Tibet, the increasingly heavy taxes sparked dissent between the people and their lords, and therefore between some clans and the king. There were calls to cease forced support of the monasteries. Ralpachen was assassinated, and his brother, U'i dum btsan po, later known as Langdarma, assumed the throne. Since the forces of the imperial army were more closely allied to their clan lords than to the central administration in Lhasa, dissent quickly became rebellion. Garrisons were pulled back from frontier posts to deal with conflict in Tibet itself and to protect clan seats and lands. Lords waged war against other lords in battles over resources, or when they saw opportunity in weakness in a neighboring area. The monasteries came under fire for the taxes that supported them in a time of famine, but in a pattern that would repeat itself over the next thousand years in Tibet, they also took sides in the rebellion and materially supported various factions and opposed others. At this time, monks were still subject to being called up for military service. Because Buddhism in Tibet at this time was still so closely aligned with the nobility, supported by it, and in many ways in the nobility’s employ, the monasteries were at the heart of the political and military turmoil. Tibetan history casts Langdarma as a villain who set about destroying Buddhism entirely in favor of Bön, but writings contemporary to the events paint a different picture. Langdarma may even have been a loyal patron of Buddhism; the truth is not entirely clear, but evidence—especially texts unearthed at Dunhuang— points to generalized conflict involving both clans and monastic bases of power rather than a royal campaign to wipe out the Dharma in Tibet.
Early in the conflict, Langdarma was assassinated by Lhalung Pelgyi Dorje; current research leans towards identifying him as the abbot of the country’s largest monastery at Samye. Obviously, the assassination of the king by the abbot of a monastery under such circumstances would have only fanned the flames of the conflict.
The royal succession at Langdarma’s death was unclear in the extreme. Neither of his two main wives had borne a child, but both claimed to be pregnant. When a child was born later to one wife, the other soon produced a child said to be her own. The boy was not a newborn, however, and his parentage was in serious doubt. Some recent research says that this child was the nephew of the senior queen and was crowned king, but this meant the divine sanction of the royal line was broken and the monarchy was fatally weakened. Clans lined up for or against one or the other queen and heir, and rebellion became open civil war.
Clan lords and their supporters hunkered down within their borders between battles. Since many of the clan nobles had aligned themselves with Buddhism and had established relationships with teachers and adepts, a number of these close connections continued into and through the Dark Age. The monasteries, as both bases and symbols of political power, came under determined attack. Tantric practitioners, however, had always been more itinerant, and with smaller groups of students gathered around them than those found in the monasteries. Many Tantric adepts had also gained the reputation of being powerful magicians, and the nobles continued their patronage for protection and divination; Tantrikas are supported throughout the Himalayas to this day for these purposes. The lords even allied with Tantrikas as a part of their arsenals.
One of the most popular Tantras in Tibet at the time was the Guhyagarbha Tantra. This text is considered the quintessential Mahayoga Tantra, and includes sections instructing the Tantrika on sgrol (pron: drol), or ‘liberation,’ along with other siddhis and aspects of realization. Interpreted figuratively or literally, sgrol refers to killing—liberating from their destructive karma—an opponent or someone who is otherwise causing suffering or unable to overcome their own suffering. The most famous example of this activity in terms of Tibetan Tantra is the story of Milarepa causing a house to collapse on and kill his corrupt relatives, but many other stories exist, including accounts of yogis wiping out entire armies. Such powers were feared and respected at the time, and were probably a major reason for the Guhyagarbha’s popularity among the king and the nobles who patronized and controlled its translation, dissemination, and practice. Whether or not they employed psychic weapons against rival armies, Tantrikas were widely supported and protected for their intercession with friendly gods and spirits, and to guard against malicious local gods and those in the service of rival lords and clans.
The small communities around these Tantrikas and their students continued their practices throughout the Dark Age. They studied, transcribed and translated texts. The state of development of written Tibetan and especially its Buddhist lexicon at the end of the Dark Age/beginning of the second spread makes this quite clear. With royal restrictions on the practice and dissemination of the Tantras removed, Lamas accepted new students, performed public ceremonies for rain or to thwart hail or drought, and in general functioned as the religious and spiritual authorities in their respective areas; this led to the apparent flourishing of Tantra in Tibet at this time. The practices of the Tantrikas and their interaction with the population and its needs through the Dark Age shaped Buddhism forever in Tibet; it was the time when Buddhism truly became a functioning reality for the Tibetan people rather than merely a court religion and an esoteric pursuit of the noble classes.
At around 1000 CE, descendants in the line of one of Langdarma’s 'sons' began a rise to power in far western Tibet. This line was eventually able to gain control of the heart of Tibet in dBus and gTsang. Buddhism was at this time acknowledged and promoted as the religion of the state, but in order to supplant existing bases of political power the new kings had to re-establish Buddhism according to their own agenda. New monasteries were founded based on new translations of newly imported texts. The existing lineages and their accomplishments were discounted by the state as not genuine, or as corrupted through time by association with the years of turmoil and war. Since Buddhism had become popular throughout the country amongst the common people as well as the nobles, serious attempts were made to maintain control by importing only Sutric, monastic Buddhism, with its strict moral code and ethos of hierarchy and renunciation, and leave the volatile Tantras out of this new formulation of the Dharma. As can be seen from the subsequent history of Tibet and Buddhism, the effort to entirely repress the Tantras failed, but new restrictions about who could practice them where and when were put in place. Entire classes of Tantra were declared banned.
Ironically, within a few hundred years the power of the monasteries had grown to the point that they gained the upper hand and turned Tibet into a theocracy, incorporating and supplanting the dominance of both the king and the powerful clans. In some ways the role of the Dala’i Lamas as incarnations of Chenrezigs was not so far from that of the god-kings of imperial Tibet, rulers descended directly from the gods and divinely ordained to rule.