How can we explain the decline of Buddhism in India around the 12th century?

There is no exact proof that explains the decline of buddhism in India. But as you mentioned this started in late 12th Century. But we can find many theories on the disappearance of Buddhism in India.

1. Influence of Brahmanism

   We all know that Hinduism is one of the most dominant and oldest religion in India. But after Great Emperor became devout Buddhist, he started flourishing Buddhism. But after the end of lineage of Ashoka's Dynasty, Brahmans who were devout Hinduism started neglecting Buddhism. There are many rumors that the first Shung King who were also devout Brahmanism, were hostile to Buddhist monks and also destroyed many monasteries, shrines, and Buddhist temples.

2. Invasions of White Nu

   You probably know more about this.

3. Destruction of Nalanda Monastery

   Nalanda Monastery was the symbol of Buddhism in that era, but it was destroyed in 1197 and that gave the momentum for the decline of Buddhism in India.

1. Corruptions in Buddhist Sanghas:

   In course of time, the Buddhist ‘Sangha’ became corrupt. The monks and followers came to be drawn towards luxury and enjoyment. Receiving and saving valuable gifts like gold and silver made them greedy and materialistic. They came to lead a life of indiscipline. Their example and perverted life-style could not but bring popular hatred. No more the people were inclined towards Buddhism.

2. Reform in Hinduism:

   Buddhism had dealt a heavy blow to Hinduism. Threatened with extinction, Hinduism started to re-organize itself. Attempts were now made to give up the complex system of rites and rituals and make Hinduism simple and attractive. This helped revive Hinduism and made it popular again. This took away the fragrance out of the flower of Buddhism. The decline of Buddhism became inevitable.

3. Division among the Buddhists:

   Buddhism faced divisions from time to time. Division into various splinter groups like ‘Hinayana’, ‘Mahayana’, ‘Vajrayana’, ‘Tantrayana’ and ‘Sahajayana’ led Buddhism to lose its originality. Also the influence of tantricism made people hate it. The simplicity of Buddhism was lost and it was becoming complex. This was enough for the people to keep away from it. The decline of Buddhism became a matter of time.

4. Use of Sanskrit Language:

   Pali and Prakrit, the spoken language of most people of India, was the medium for the spread of the message of Buddhism. But Sanskrit replaced these at the Fourth Buddhist Council during the reign period of Kaniska. Sanskrit was a complex language, hardly understood by common people. It was the unintelligible Sanskrit language that had accounted for the decline of Hinduism, earlier. Now, when Buddhism adopted that language, few people were able to understand it. People rejected what they could not understand.

5. Role of Hindu Preachers:

   Harsavardhan drove away the Brahmins from the religious council held at Kanauj. These Brahmins, under Kumarila Bhatta, fled to the Deccan. Under Bhatta’s leadership, Brahmanism staged a come-back. Adi Sankaracharya also revived and strengthened Hinduism. He defeated Buddhist scholars in religious discourses which were held in many places in course of his tour of the whole of India.

   Thus, the superiority of Hinduism over Buddhism was established. This trend continued through the efforts of Ramanuja, Nimbarka, Ramananda etc. Hinduism regained its lost glory, position and popularity. It came to be at the expense of Buddhism.

6. Rifts in Buddhist Order:

   The internal rifts and divisions in Buddhist order made the rise of any new apostle impossible. The earlier examples of Ananda, Sariputta and Maudgalayana became very rare. The spirit and missionary zeal of Buddhism was lost for ever. Thus, the decline of Buddhism came in the absence of dynamic preachers and reformers.

7. Buddha Worship:

   Image worship was started in Buddhism by the Mahayana Buddhists. They started worshipping the image of the Buddha. This mode of worship was a violation of the Buddhist principles of opposing complex rites and rituals of Brahminical worship. This paradox led the people to believe that Buddhism is tending towards the fold of Hinduism. Buddhism’s importance decreased thereby.

8. Loss of Royal Patronage:

   In course of time Buddhism came to lose royal patronage. No king, worthy of note, came forward to sponsor Buddhism after Asoka, Kaniska and Harsavardhan. Royal patronage works magically for the spread of any faith. Absence of any such patronage for Buddhism came to pave the way for its decline in the end.

9. Huna Invasion:

   The ‘Huna’ invasion jolted Buddhism. Huna leaders like Toamana and Mihirakula opposed non-violence completely. They killed the Buddhists residing in the north-western part of India. This frightened the Buddhists of the region either to give up Buddhism or go into hiding. None dared to spread the message of the Buddha during those times. As a result, Buddhism became weak and depleted.

10. Emergence of Rajputs:

    Emergence of the Rajputs became an important reason for the decline of Buddhism. Kings of such dynasties as Bundela, Chahamana, Chauhan, Rathore etc. were militant rulers and loved warfare. They could not tolerate the Buddhists for their message of non-violence. The Buddhists feared persecution from these Rajput rulers and fled from India. Buddhism became weaker and faced decline.

11. Muslim Invasion:

    The Muslim invasion of India almost wiped out Buddhism. Their invasions of India became regular and repeated from 712 A.D. onwards. Such invasions forced the Buddhist monks to seek asylum and shelter in Nepal and Tibet. In the end, Buddhism died away in India, the land of its birth. 12th century Kashmir kings were said to have persecuted Buddhist.

12. Bhakti movement:

    Along with Shankaracharya and other reformers, Alwars and Nayanars started portraying devotion from Tamil Nadu (from 6th century CE), which spread all over India slowly. With this Bhakti movement, Hinduism got influenced and other faiths like Jainism and Buddhism started declining.

Conclusion: both Hindu reformers and Muslim rulers played a vital role in the decline of Buddhism in India.

Note:(1) This is a very controversial topic in India and Indian diaspora and India is ruled currently by Far- Right with a huge voter base and a lot of times in the history of India far- right people has been at the helm of the nation in bureucracy, politics and in Academia.(2) So, I request the Moderators to protect the answer from defiling by rouges.(3) Any bad comment or comments in poor taste will not be replied to.

"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).

Ambedkar says:

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).

Buddhism declined in India, due to assimilation.

Firstly, Buddhism had flourished in ancient India, Pakistan and Afghanistan, but eventually, the Buddhist locals became subject to "hard" assimilation by foreign invaders from Muslim lands. See the Wikipedia articles Buddhism in Afghanistan, Buddhism in Pakistan and also this article section. This is not unlike the disappearance of Zoroastrianism from its homeland of Persia.

Secondly, Buddhism suffered from "soft" assimilation into Hinduism.

Before Buddhism (around 500 BCE), the historic Vedic religion was mainly focused on sacrifices and rituals. It did not have robust philosophy and metaphysics. After encountering Buddhism and the shramanic movement (which includes Jainism), the Upanishadic movement rose in India from 500 - 200 BCE. After this, the Vedanta movement rose as part of Classical Hinduism from 200 BCE - 1100 CE. The Hindu text Brahma Sutras that attempted to refute Buddhist and other non-Hindu teachings, was written between 200 BCE and 400 CE. These scholarly dates can be verified by referring to Wikipedia.

Adi Shankara, the famous reformer who revived Hinduism and consolidated Advaita Vedanta, lived around 800 CE. Advaita is the philosophical school of Vedanta that is closest to Buddhism. Even in those days, Hindu philosophical opponents of Shankara accused of him of being a crypto-Buddhist. Through Advaita Vedanta, Hinduism assimilated Buddhism by adapting parts of its philosophy unto itself. Some Hindus even claim that Mahayana's emptiness (shunyata) is exactly the same as Advaita's Transcendental Ultimate Reality.

On the other hand, the Dvaita Vedanta school of Hinduism, which has completely incompatible philosophy to Buddhism, assimilated Buddhism by making Buddha an incarnation of the Hindu deity Vishnu. The Hindu text Bhagavata Purana written between 500 CE and 1000 CE states that Vishnu incarnated as Buddha to delude the immoral atheists into living ethical lives. According to them, the Buddha tricked atheists into taking refuge in the Buddha and serving the Buddha, who is actually God in disguise. More info on this is available on this page.

The final reason for Buddhism's "soft" assimilation into Hinduism may be the development of concepts in Mahayana Buddhism that sound very similar to concepts in Hinduism, like the Eternal Buddha, Adi Buddha (Primordial Buddha), Buddha Nature (tathagatagarbha), storehouse consciousness (alayavijnana) and beings like Avalokiteshvara etc.

Buddhism survived in the world through three routes - Tibetan Buddhism (and the preservation of the Tibetan Buddhist canon), East Asia (through transmission of Mahayana texts to China) and Theravada Buddhism's revival through Buddhism's own version of Adi Shankara, Buddhaghosa.

Buddhism has experienced a limited revival in India due to three reasons. The first is the conversion of low caste Dalits to Buddhism by Dr. Ambedkar. The second is the popularization of Vipassana meditation by S. N. Goenka. The third is the relocation of the Dalai Lama to Dharamsala, India.

In his book Charles Allen argues that with the fall of Ashokan Empire, Buddhism, the then official religion of the king started to decline. Also the decline in Buddhism started the revival of Hinduism by Pusyamitra Sunga who was a Brahmin from Bhardwaj Gotra, one of the highest castes in Hinduism.I'd also like to add that from 6/7th Century onwards India witnessed the Bhakti and Sufi Movements which also helped in reconverting the converted Buddhists into Hinduism an Islam .

There were a multitude of complex factors that led to the decline of Buddhism around the 12th Century.

But first some caveats:

1. Buddhism is/was not a monolithic entity- there are many "Buddhisms".
2. No religion, culture, social custom has ever died out completely in India, right from the times of Mohenjodaro- they have lived on in new forms in new homes.
3. All religions in India have borrowed from other religions.

Now the short answer:

1. Buddhism became increasingly complex over the centuries as the philosophies got refined. (eg. Abhidharma, Sautrantika, Madhyamaka, Yogacara). This took it out of popular culture and understanding.

2. Buddhism increasingly became a religion of renunciation and asceticism. So if you did not want to renounce worldly life and become a monk, you wouldn't probably take up Buddhism. Add to that the fact that the other sects/cults started mocking the monks' way of life as lazy and beggarly - merely an excuse to avoid the hardships of life.

3. The ethos of non-violence was not compatible with the militaristic expansion policies of the kings. So they could not favor or promote Buddhism. Without royal patronage it would be difficult for any religion to prosper. This was especially true of the Rajputs.

4. Many regional languages came into being, and along with that local deities, local heroes, texts and customs. For example Marathi came into existence, and Marathi devotional poems promoted local deities. Similarly Tamil devotional poems had profound impact through love/devotional "Shangam" poetry. Kannada had royal patronage, and became the language of the Lingayat movement. Local deities outside the Puranic tradition, such as Panduranga/Vitthala gained acceptance- they were semi-gods/heroes. This made the non-elite/non-Brahmin acceptable as god. Many such local heroes attracted local hymn writers such as Namdeva, Sena, Narahari and Janabai, who were people with ordinary professions in their day to day life. Janabai was a maid servant, Namadeva a tailor and Sena a barber. All these movements were thus converging towards a democratization and popularization of theology.

5. Many of the local texts and hymns, such as of the Alvars and Nayanars blurred the distinction between monarchs and deities. This suited the monarchs and their political agenda. There are complex explanations of the equation of the state with the temple and the monarch with the deity. "The king is seen as the focus of loyalty and demands devotion from the intermediaries and the subjects, while the deity receives similar sentiments from the worshipers. This implicit overlap may have encouraged the rulers to patronize the devotional sects, underlining the notion of loyalty."

6. Temples became political extensions and therefore powerful centres, attracting the masses. They served the dual purpose of extending monarchial supremacy and serving to use Puranic ideology to establish caste system in places where they did not exist. Note that this was politically expedient to provide a stable social structure for the efficiency of state building.

7. The Bhakti movements simplified religion for the masses. The common man could now relate "personally" to God and did not need the intervention of the priest. The ability of an "individual" to search for liberation outside the orthodox rituals made these movements popular and acceptable. Therefore many flavors of Bhakti cults grew. By the end of this period Vaishnava, Shaiva and Shakta cults became more dominant. Kashmir Shaivaism/Tantra also came into prominence. Then there were the Kapalika, Kalamukha, Bhagavata and Pashupata cults to name a few. Far too many sects were prevalent in the various regions of the subcontinent. In this hectic competition, anyone could have "won" in the end. But what can be said with certainty is that Buddhism's dominant role as opposer of orthodox Vedic Brahmanism was usurped by the newer cults. Borrowing from Buddhist ideas, even the local deities became "compassionate" deities. This too, somewhat snatched from Buddhist "compassion" ideas and legitimised other sects as equally 'peaceful'/benevolent.

8. Besides the Bhakti cults, Dwaita, Advaita, and other schools of thought came into existence. The effects of revivalism through thinkers such as Shankaracharya, Ramanuja and Madhava started paying dividends as the old order of Vedic Brahmanism was destroyed to form new theological doctrines. Interestingly, everyone borrowed freely from everyone else to be able to compete. Examples: Buddha became accepted as the last avatar of Vishnu. And Madhava's philosophy, that only the devoted would be salvaged seems to have been borrowed from the Malabar Christians. Tantras influenced Buddhism too- this became the Vajrayana Buddhism. But possibly, austere Buddhism borrowed less- and therefore the religion "stagnated".

9. Puranas, Mahabharata and Ramayanas grew in popularity in local languages, leaving the Jataka tales of Buddhism behind. Possibly these were not translated from Pali. (eg Kamban's Ramayana, as opposed to Valmiki's, is quite sympathetic to Ravana- but the point is that local acceptance of non-Buddhist tales/mythology/fables had an impact on regional culture.)

10. A few years later the arrival of Sufis from Persia, and more specifically their borrowing, intermingling and exchange of ideas with the Bhakti movements of Punjab and Bengal also infused new thinking. Arrival of Jews, Zorashtrians and Christians also added to the growing chaos.

11. Last, and probably the last nail in the coffin- the routing of Buddhism by the Mohammedans from the west. This happened a while later, but cleared up the remaining influences of Buddhism from the North western provinces.