Three great monasteries
Anuradhapura held three enormous monastic complexes. The Mahāvihāra was the oldest, founded at the time of the first mission. The Abhayagiri was established in the 1st century BCE and the Jetavana later, and both were more open to new texts and ideas, including Mahāyāna ones, than the Mahāvihāra approved of.
Royal favour moved between them for centuries, and so did arguments about discipline and scripture. In the 12th century Parākramabāhu I reorganised the whole order under the Mahāvihāra's line. That is the reason the Theravāda the world inherited is the Mahāvihāra's version of it.
Writing it down
The chronicles record that the canon was committed to palm leaf at Aluvihāra in the 1st century BCE, in a time of famine and war, for fear that the memorised transmission would be lost. The commentaries were kept in Sinhala, and later translated into Pāli so that the whole tradition would be readable across the Buddhist world.
Sri Lanka also produced the Buddhist world's most sustained historical writing: the Dīpavaṃsa and then the Mahāvaṃsa, which tell the island's history as the history of the teaching in it.
Breaks and repairs
Invasion, famine and dynastic collapse repeatedly reduced the number of fully ordained monks, sometimes to the point where ordination could not be conferred at all. Each time, the lineage was restored by inviting monks from abroad: from Burma in the 11th century, and from Siam in 1753, after Väliviṭa Saraṇaṅkara's reform, which founded the Siyam Nikāya that is still Sri Lanka's largest monastic order.
Across the same centuries the Buddha's tooth relic became the emblem of legitimate kingship. Whoever held it held the right to rule, which bound the monastery and the throne tightly together.
Why it matters
Sri Lanka's Theravāda tradition would later shape Buddhism across mainland Southeast Asia.
History and tradition
What historians can say
The ruins and inscriptions of Anuradhapura, a UNESCO World Heritage site, confirm the scale of the monasteries and the royal gifts that kept them. The chronicles are religious history written by Mahāvihāra monks: they report real events, and they report them with a firm view of who was orthodox.