What happened
Ghurid armies moved into northern India at the end of the 12th century, and the forces of Bakhtiyar Khalji reached Bihar around 1200. The Persian chronicle Tabaqāt-i-Nāṣirī, written in the 1260s, describes the storming of a fortified college whose shaven headed inhabitants were killed, and a library of books that no one left alive could read. It is generally taken to be Odantapurī.
Nālandā and Vikramaśīla were destroyed or abandoned in the same period. The Tibetan monk Dharmasvāmin visited Nālandā in the 1230s and found a handful of teachers living among ruins, with a few students, afraid of soldiers nearby.
What it did not do
It did not by itself end Buddhism in India. The centuries before had already thinned the lay community and shifted the patronage, which is why these losses were final rather than survivable. Buddhist communities continued in the Himalayas, in Nepal, in Bengal and in the south for generations afterwards.
Nor was this a religious war with Buddhism as its target. Hindu and Jain temples were plundered in the same campaigns, wealth being the point, and some Muslim rulers elsewhere in Asia later patronised Buddhist institutions. Historians such as Johan Elverskog caution against reading the period as a straightforward collision between Islam and Buddhism.
Afterwards
Teachers and manuscripts moved north to Tibet and Nepal, where the lineages they carried continued and in some cases survive only there. Bodh Gaya was maintained with Burmese and Sri Lankan help over the following centuries.
Buddhism returned to India as a public presence in the 20th century: through the Maha Bodhi Society, through Tibetan refugees after 1959, and through B. R. Ambedkar's conversion in 1956, which brought hundreds of thousands of Dalits into Buddhism with him.
Why it matters
By the later medieval period, Buddhism survives far more strongly outside India than within much of its birthplace.
History and tradition
What historians can say
The evidence is a small number of chronicles, burnt layers at the sites themselves, and Dharmasvāmin's travel account. Nālandā's remains are a UNESCO World Heritage site. Numbers of dead are not recoverable, and the round figures sometimes quoted come from much later retellings.