By then , the Pagan Empire , despite outward appearances of calm , had been in long and slow decline since the early 13th century . The continuous growth of tax @-@ free religious wealth had greatly reduced the tax base of the kingdom . The crown had lost resources needed to retain the loyalty of courtiers and military servicemen , inviting a vicious circle of internal disorders and external challenges . Although it was able to put down the first batch of serious rebellions in 1258 – 60 in South Arakan and Martaban ( Mottama ) , the decline continued . On the eve of the Mongol invasions , between one and two @-@ thirds of Upper Burma 's cultivable land had been donated to religion . The crown 's ability to mobilize defenses was in serious jeopardy .