Task: Question Generation
Input (English): Passage: Kublai essentially demanded that Japan become a vassal and send tribute under a threat of conflict. However, the emissaries returned empty-handed. A second set of emissaries were sent in 1268, returning empty-handed like the first. Both sets of emissaries met with the Chinzei Bugyō, or Defense Commissioner for the West, who passed on the message to Shikken, Hōjō Tokimune, Japan's ruler in Kamakura and to the Emperor of Japan in Kyoto. After discussing the letters with his inner circle, there was much debate, but the Shikken had his mind made up; he had the emissaries sent back with no answer. The Mongols continued to send demands, some through Korean emissaries and some through Mongol ambassadors on March 7, 1269; September 17, 1269; September 1271; and May 1272. However, each time, the bearers were not permitted to land in Kyushu. The Imperial Court suggested compromise, but really had little effect in the matter, due to political marginalization after the Jōkyū War. The uncompromising shogunate ordered all those who held fiefs in Kyūshū, the area closest to the Korean Peninsula and thus most likely to be attacked, to return to their lands and forces in Kyūshū moved west, further securing the most likely landing points. After acknowledging its importance, the Imperial Court led great prayer services, and much government business was put off to deal with this crisis.
Output (English): Over how many years did the Mongols send demands through Korean emissaries or Mongol ambassadors?