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"How can we explain the fall of Buddhism in India, which is the geographic origin of Buddhism?
Were there ever any direct conflicts between Hinduism and Buddhism?"
Brahmin caste and their supporting castes were responsible for the fall of Buddhism in India. I will support my this argument with sources from History and Philosophy books.
Excerpts from the book Debrahmanising History, by Braj Ranjan Mani , Chapter 4 , Chapter Name: Buddhist India : The mauryans were not interested in giving brahmans preferential treatment and certain immunities that they demanded as their birth right. Ashoka introduced the policy of equality in civil law and criminal law for everyone( Page 129-130).
In another edict, Ashoka exposed the Bhudevas ( gods on Earth ) as false gods. In Ashoka's rule there was indifference to Sanskrit, the language of Brahmanic Priestcraft. Such a policy hit the interests of the brahmins who depended on the donations and gifts given to them in various rites and sacrifices. In other words, the superiority of the brahmans and the credibility of the sacerdotal literature were under threat.
The Brahmins retaliated. Led by Pushyamitra Sunga, the commander of the last mauryan king Bridhadratha, they hatched a conspiracy. Pushyamitra gave his daughter in marriage to the king, got coveted post and cold - bloodedly beheaded his master during a military parade in 185 BCE... After burning monastries in and around Patliputra he went up to Sakala and offered a reward of 100 dinars or gold pieces for the head of every Buddhist monk.... That he unleashed a reign of terror against Buddhists is corroborated by several sources including the Tibetan and Chinese records. From Chinese authorities it is known that many Buddhists still do not pronounce the name of Pushyamitra without a curse( cited in Ambedkar, ibid). Rejecting the established trajectory of Indian history, Ambedkar in his incomplete work The Revolution and Counter Revolution in Ancient India has made the conflict between the Buddhism and Brahminism the corner stone of restructuring the past. ( K.P. Jayswal, too in his Manu and Yajnavalkya [1930: 40-1], has termed the Brahmanical reaction the 'orthodox counter -revolution'). Ambedkar's contention that the process of the decline of Buddhism in India was a violent one (not a process of gradual adjustment and absorption into Hinduism as is routinely portrayed by the elite historians ) has been corroborated by ample archaeological and textual evidences in Verardi's brilliant work. (2011) ( Page 130- 131).
Now if one bears in mind the fact that the Brahmanic Revolution by Pushyamitra took place in 185 BCE there remains no doubt that the code known as Manusmriti was promuglated by Pushyamitra as embodying the principles of Brahmanic Revolution.
Page 132: Ambedkar cites some verses from the Manusmriti to show the author's antipathy to Buddhism... Ambedkar sees parallel between the Muslim invasions of Hindu India" and Brahmanic invasions of "Buddhist India", the Brahmanic invaders of Buddhist India, the Shungas, the Kanvas and the Andhras fought among themselves for superiority but had one common goal to destroy Buddhism.
In his analysis of destruction of Buddhism , Ambedkar also mentions a "subversion theory". the subversion involved the insidious role of brahman - Buddhists. The higher philosophies of both Buddhism and brahmin began to converge in essence. In other words , they continued to be brahmins despite accepting Buddhism and probably observed their caste excluvism as well. They produced doctrines akin to the metaphysics of Brahmanism which culminated in the Mahayana school...
Page 133: The Mahayanists transformed the Buddha into a god, introduced his several incarnations... even tried to co-opt Buddha in the Brahmanical mould by depicting him as the ninth avatar of Vishnu.
Ambedkar Says: " The history of India is nothing but a history of mortal conflict between Buddhism and Brahmanism." ( Book: Revolution and COunter Revolution in Ancient India).
Research by Swami Dharma Teertha in his book History of Hindu Imperialism also sheds light on conflict between Buddhism and Hinduism. Kings like Harsh Vardhan, Kansihka who supported Buddhism were killed by Brahmins and their supporting castes by hatching conspiracies.( Source: The above mentioned book by Swami Dharma Teertha).