expeditions to Arakan to place its claimant Letya Min Nan on the Arakanese throne. The first expedition failed but the second in 1118 succeeded. Sinhalese Mahavamsa Chronicles mention a dispute over trade in elephants between Burma and Sri Lanka that prompted the Ceylonese forces to raid Lower Burma in 1180. The authority of Pagan continued to grow, peaking in the reign of King Narapatisithu (r. 1174–1211). The king formally founded the Royal Palace Guards in 1174, the first extant record of a standing army. Pagan's influence reached further south into the upper Malay peninsula, at least to the Salween river in the east, below the current China border in the farther north, and to the west, northern Arakan and the Chin Hills. Up to the first half of the 13th century, much of mainland Southeast Asia was under some degree of control of either the Pagan Empire or the Khmer Empire. Late 13th century rebellions The Pagan military power was closely intertwined with the kingdom's economic prowess. The kingdom went into decline in the 13th century as the continuous growth of tax-free religious wealth—by the 1280s, two-thirds of Upper Burma's cultivable land had been alienated to the religion—affected the crown's ability to retain the loyalty of courtiers and military servicemen. This ushered in a vicious circle of internal disorders and external challenges by Mons, Mongols and Shans. The first signs of disorder appeared soon after Narathihapate's accession in 1256. The inexperienced king faced revolts in Arakanese state of Macchagiri (present-day Kyaukpyu District) in the west, and Martaban (Mottama) in the south. The Martaban rebellion was easily put down but Macchagiri required a second expedition before it too was put down. The calm did not last long. Martaban again revolted in 1285. Mongol invasions This time, Pagan could not do anything to retake Martaban because it was facing an existential threat from the north. The Mongols demanded tribute, in 1271 and again in 1273. When Narathihapate refused both times, the Mongols systematically invaded the country. The first invasion in 1277 defeated the Burmese at the battle of Ngasaunggyan, and secured their hold of Kanngai (modern-day Yingjiang, Yunnan, 112 km north of Bhamo). In 1283–1285, their forces moved south and occupied down to Tagaung and Hanlin. The king fled to Lower Burma in 1285 before agreeing to submit to the Mongols in June 1286. Soon after the agreement was finalised, the king