and Myinsaing on 25 January. But they could not break through. The besiegers took the bribes given by the three brothers, and began their retreat on 6 April. But the commanders were executed by the Yunnan government when they got back. The Mongols sent no more invasions, and withdrew entirely from Upper Burma on 4 April 1303. First Shan raids The Mongols left but the Shan people, who had come down with the Mongols did not—just as the Burmans who came down with the Nanzhao invasions stayed behind four centuries earlier. The Shans built an array of small states in the entire northwestern to eastern arc of central Burma, thoroughly surrounding the valley. They continued to raid the Irrawaddy valley throughout the 14th century, taking advantage of the split of Myinsaing into Pinya and Sagaing kingdoms in 1315. Starting in 1356, then the most powerful Shan state of Mogaung (in present-day Kachin State) began a series of sustained assaults on central Burma. In 1364, its forces sacked both Sagaing and Pinya in succession, and left off with the loot. The power vacuum did not last long. In the same year, Thado Minbya, a Sagaing prince, emerged to reunify Upper Burma and founded the Ava Kingdom. Nonetheless, Shan raids into Upper Burma continued off-and-on in spurts. The raids were led by different Shan states at different times. Mogaung's devastating raids (1359–1368) were replaced in the 1370s and 1390s by Mohnyin's raids that reached as far south as Sagaing. After Ava conquered Mohnyin in 1406, the mantle was picked up by Theinni (Hsweni) which raided Avan territory from 1408 to 1413. The intensity and frequency of the raids lessened in the 15th century due to both the rise of Ava and indeed the arrival of the Ming Chinese in Yunnan in the 1380–1388. Chinese expeditions against Shan states Though hemmed in by two powerful kingdoms, the Shan states were able to create a space for themselves for another century and a half, ironically by using state-of-the-art Chinese military technology. Chinese firearms reached northern mainland Southeast Asia by way of Chinese traders and renegade soldiers, who despite the Ming government's prohibition, actively smuggled primitive handguns, gunpowder, cannon and rockets. True metal barrelled handguns, first developed in 1288, and metal barrelled artillery from the first half of the 14th century had also spread. The Shans soon learned to replicate Chinese arms and military techniques,