In whatever countries Buddhism has became official ideology—whether Theravada Buddhism in Southeast Asia or Tantric Buddhism in Tibet or East Asia—war has often been zealously waged. At present, the Buddhists of Sri Lanka, for example, have openly taken up the struggle against the Tamil freedom fighters. What is true of Japanese Zen holds equally for other forms of Buddhism. Long before its lyrical metaphysical flights exerted their charm, Buddhism took hold first and foremost as a tool for protecting States.
Buddhist law often had to bow to reason of State. But in many instances it also provided an ideology for counterforces, inspiring peasant revolts in the name of a millenarianism centered on the coming of the future Maitreya Buddha. In one of these movements, in China, arising at the start of the sixth century c.e., the rebels, using the Buddhist title of "Grand Vehicle" (Mahayana), undertook to rid the world of its "demons"—starting with the era's Buddhist clergy.
In Japan, on the other hand, Buddhism managed to pave the way for feudal struggles, creating a new type of religious figure, the "warrior monk." It is only at the end of the sixteenth century, after centuries of internecine struggles, that the great monasteries were subdued by the military government. The ensuing subordination explains in part why, after the Meiji Restoration (1868), Japanese Buddhism proved no force against militarism, and fell into line with "spiritual mobilization."
Thus, Japanese militarism blended Buddhist doctrine with the imperial sauce, reducing it to its simplest expression, to bend it to official propaganda. The Buddhist theory of selflessness served, for instance, to justify giving one's life for the Emperor, while the notion of the Two Truths (conventional and ultimate) served to explain the contradiction between the principle of respect for human life and patriotic duty. However, these ideas are not merely belated deviations in the necessary adaptation of Buddhism to Japanese culture. They have a long history.
In fact, reasons for bending the principle of nonviolence were never wanting. There were considerations of a practical nature: when Buddhist Law is threatened, it is necessary to ruthlessly fight the forces of evil. Kill them all, and the Buddha will recognize his own. Murder in this case is piously qualified as "liberation," since the demon, duly killed out of compassion, will be released from its ignorance and can then be reborn under better auspices. The crucial moment in Tibetan ritual dances comes when the priests stab an effigy personifying the demon forces. This ritual is thought to repeat a monk's murder of King Glang dar ma (842), a persecutor of Buddhism (as such, clearly "possessed" by Evil). Various other theories use this same casuistry, including the idea that it is just to kill out of charity or compassion, to prevent another person from comitting evil.
http://www.sangam.org/articles/view/?id=118
