Geiger on Sri Lanka's diplomatic relations with Rāmañña in Lower Burma during the Pagan period

Wilhelm Geiger's Culture of Ceylon in medieval times (1960) provides a nice succinct overview of relations between Sri Lanka and the Mon kingdom of Rāmañña during the Pagan era (citations are to Geiger, 1953):

"Rāmanñña was the name of the province of Pegu in Southern Burma. Its inhabitants were, like those of Ceylon, Buddhists of the Theravadan school. Between the two countries there had never been dissension up to the 12th century, and their monarchs were wont to send each other many costly gifts and in this way to maintain a friendly intercourse (76.10 sq). Vijayabahu I (1059-1114 A.C.) sent envoys with various presents to the king of Rāmañña and received in return valuable gifts from him (58.8 sq). When in Ceylon the number of Bhikkus had decreased so much that it became impossible to fill the chapter in order to perform the Vinaya ceremonies, the same king fetched from Ramanna Bhikkus who were thoroughly versed in the Buddhist precepts and able to restore the Order which had declined in Lanka (60.4 sq.)…

The friendship between the two countries was severely disturbed in the twelfth century. The war made by Parakkamabahu on the Rāmañña king was, according to the chronicle, successful for the Singhalese. The former friendly relations were restored by King Vijayabahu II, 1186-87. The King himself composed a letter in the Maghada language - i.e. Pali, the lingua franca among the Buddhists - which he sent to the ruler of Rāmañña and concluded a treaty with him as Vijayabahu I had done before (80.6-7). (Geiger and Bechert, 1960, 134-135).

Trade disputes are said to have been a motivating cause behind the Singhalese raid on Lower Burma:

"In the medieaval period various stuffs, sandal-wood, camphor and the like were imported from Rāmañña, Southern Burma (58.9 sq.). From the same country elephants were brought to Ceylon though plenty of them roamed wild in the forests. The fact that in the twelfth century the king of Rāmanñña tried to monopolize the elephant-trade and enormously raised the prices, was one of the reasons by which Parakkamabahu's war with Rāmañña was provoked (76.17-34). (Geiger and Bechert, 1960, 108)

References

Geiger, Wilhelm and Christian Mabel Duff (trs.) (1953) Culavamsa : Being the more recent part of the Mahavamsa, Colombo, Sri Lanka: Ceylon Govt. Information Dept.

Geiger, Wilhelm (1960) Culture of Ceylon in medieval times, ed. Heinz Bechert, Wiesbanden: Otto Harrassowitz