Why did Yuan Shizu expedition to Japan and Annan during Kublai Khan's reign?

1260, Kublai Khan preached Mongolian Khan in Kaiping. During his reign, he continued to expand his territory and made expeditions to Japan and Annan (now Vietnam). There are three reasons for exploring.

First of all, deeply influenced by culture, Kublai Khan was deeply influenced by the feudal system of grassland. The Yuan Dynasty unified the division of China for nearly a hundred years and laid the foundation for the present territory of China. Later, the Ming and Qing dynasties continued to maintain and consolidate. However, faced with such a vast territory, Kublai Khan, the emperor of the Yuan Dynasty, was not satisfied. As a descendant of Genghis Khan, he has been adhering to the idea of conquering all directions and seizing everything. The ambition of Mongolian Yuan rulers to dominate the Quartet was deeply reflected in Mongolian Khan. The descendants of Genghis Khan embodied the strength of the Mongolian nation and maintained the authority of the Mongolian Khan in four expeditions. Through the election, they can be regarded as feudal patriarchs, clan relatives and heroes, and at the same time they can get the support of their own country. Kublai Khan, who established the Great Yuan Empire, was opposed by many Mongolian kings to his throne. He urgently needs to transfer domestic political contradictions through war and acquire land and people through war and enfeoffment to maintain his prestige.

Secondly, convenient maritime transportation and rich experience in naval warfare provide preconditions for foreign countries. With the gradual expansion of the navy and the convenience of maritime traffic, Kublai Khan followed the ancestral training of territorial expansion, and launched wars against Asian countries when the ambitions of many foreign envoys in the Yuan Dynasty did not receive due response. In view of the hopeless western expedition, East Asia and South Asia have their own advantages, and Kublai Khan aimed at East Asia and Southeast Asia that had not been conquered by Mongolian cavalry.

Finally, there is another reason, that is, to hunt down the immigrants in the Southern Song Dynasty and prevent them from relying on the strength of other countries to recover their lost land. After the demise of the Southern Song Dynasty, Kublai Khan was very distrustful of the troops left over from the Southern Song Dynasty. Some people fled from China to Annan. Kublai Khan was worried about the harm of the remnants of the Southern Song Dynasty, so he kept going south.