The history of Vajrayana, its various roots and its development in Tibet, has often been conveyed in a way that makes it appear to have consisted of four easily defined periods:
In one sense, there's little wrong with this representation; it does cover most of the contributing elements that led to this specific, Tibetan development of Buddhism. In another sense, viewed without any sectarian bias and making use of the historical and linguistic research available at the beginning of the 21st century, some of these four steps in the above representation are under-represented or simplified.
The following account - divided into seven stages rather than four - provides the reader with a more comprehensive view of the development of Vajrayana (the Tibetan form of Mahayana Buddhism influenced by both Bön and Tantra) by showing that the first diffusion was not really the first, that the second diffusion (actually the third) resulted in something other than only the so-called "four schools", and that the transition from the periods 3 to 4 above was dominated more by struggles for political power than by subtle questions of religious doctrine.