Task: Question Generation
Input (English): Passage: At Pagan, the new king Kyawswa controlled just a small area outside the capital. Instead, the real power rested with three former Pagan commanders of nearby Myinsaing. It was the brothers, not the nominal sovereign Kyawswa, that sent a force to retake Lower Burma in 1294 after Wareru had decided to become a vassal of the Sukhothai Kingdom. Wareru drove back the Myinsaing forces. Concerned by the increasing power of the three brothers, Kyawswa submitted to the Mongols in January 1297, and was recognised by the Mongol emperor Temür Khan as viceroy of Pagan on 20 March 1297. In December 1297, the three brothers overthrew Kyawswa, and founded the Myinsaing Kingdom. In January 1300, the Myinsaing forces led by Athinhkaya seized the southernmost Mongol garrisons named Nga Singu and Male, just north of modern Mandalay. On 22 June 1300, the Mongol Emperor declared that Kumara Kassapa, one of Kyawswa's sons, was the rightful king of Burma, and sent in a 12,000-strong army from Yunnan. The Mongol army reached Male on 15 January 1301, 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.
Output (English): Who was concerned by the increasing power of the three brothers?