-The other problem was Japan itself. In the 1300s, the Kamakura Shogunate that ruled Japan was losing its grip on the country, leading to local lords acting on their own initiative and power struggles in Kamakura itself. In addition to the loss of trade with China due to the chaos of the Mongol Invasions, a lot of Japanese lords became impoverished due to the slack in trade. As such, Nips took to piracy and plagued the coastlines of Japan, Korea, and China throughout the 1300s. The Kamakura Shogunate proved too inept to handle the threat, especially since a lot of coastal Jap lords were in on the piracy issue, with their own Samurai going on these raids.
As the Yuan Dynasty was trying to secure its legitimacy as a Chinese Imperial Dynasty- and also to restart trade in East Asia- the Mongols therefore saw pirates as fucking headaches.
- And so, with the Yuan therefore sent emissaries to tell Japan to recognize that its a tributary to the Great Yuan Empire, to recognize the Mongol Emperor as Emperor of China, for Japan to send appropriate tribute of slaves/treasures (and not the pleasant gifts they exchanged with the Chinese & Japanese monarchs in previous dynasties), and for Japan to stop piracy in the region.
When the Mongs ambassadors made these demands to Japan, they made it with such arrogance (OG Chinese Tributary missions were courteous "hi-hello" affairs between Monarchs) that it fucking insulted the Kamakura Shoguns. Coupled with the idignity of people they considered "barbarian" steppeniggers treating Japan as such, and the threats that the Ambassadors issued to Japan, the Shoguns had the Ambassadors beheaded, continued considering the Yuan as niggerous barbarian usurpers of the Chinese throne, and refused to become a tributary of Yuan China. Thus triggering the Mongol Invasions of Japan.