To recover some of this wealth in an acceptable manner, kings often saw fit to "purify" or reform the Buddhist sangha (monastic community). However in the 13th century, no Burmese kings were strong enough to manage and reform the increasingly rich and powerful sangha. The situation was also compounded by drier weather during the late 13th century and the 14th century (the Medieval Warm period), which lowered crop yields. Because of this, the state was weak and divided. It was unable to resist the invasion of new enemies like the Mongols, Hanthawaddy and the Shans.