Tibetan history can only reliably be traced back to about the C6th CE; before that, evidence is too fragmentary to be trusted. It is apparent that early religious beliefs there were based on a shamanistic system known as Bon. This was subsequently combined with the Buddhism brought from India in the south and China in the east to form the Lamaism that continues today as a form of religious authoritarianism.
Tibet has always been on the outskirts of Chinese society. The Han Chinese have created and sustained a sedentary, agricultural society based on the eastern coast and central plains region. Around their area, they have been surrounded by nomadic or semi-nomadic peoples who have represented a threat to them and to their way of life. The Tibetans represent one of these peoples. They had access to horses and trade goods from the west, by means of the Silk Road, which were of great interest to the Chinese. The Chinese themselves maintained an organized and civilized society that provided many goods of value to the outsiders. They also provided a non-moving target that the fast moving horseback riders could raid with little fear of reprisals.
The film 'Crouching Tiger, Hidden Dragon' illustrates some characters from the outside world - in that case, the outsiders were Turkic tribespeople from the deserts of Xinjiang or somewhere similar - but the portrayal is similar. That is, exotic and sexy outsiders, with strange and slightly dangerous customs. Horses, movement and freedom dominate the scene - the ideas could be transplanted to a much more modern setting, of course.
In any case, the Chinese knew the early Tibetans as the Qiang, who were a Mongolian people who remained nomadic in the north but settled on the Tibetan plateau in the southern part of their territory. The northern part was a part of the vast steppes of eastern mainland Asia that has seen the rise of a large number of nomadic tribes. The Huns, the Magyars, the Bulgars and the Khazars, among others, first came to prominence from some part of this region. A combination of factors combined to cause some kind of population pressure that led them to move away from their homeland, conquering the intervening peoples in their way until they were able to establish new home territories - or defeated. West of China, they formed a large cattle-dealing and breeding and caravaneering society.
The Qiang on the Tibetan plateau were divided into three separate clans and were not united until 620, at which time they were able to combine their strength and look to their neighbours for expansion. Tibetan armies were based on cavalry and, specifically, heavily armoured cavalry capable of powerful charges aimed at intimidating enemy infantry or opposing lighter armed cavalry into scattering. Under Song-tsen Gampo, Tibetains defeated the Tuyuhan nomads around Lake Kokonor and went on to attack Chinese Sichuan province.