Alternate Strategy
Hello everyone,
After looking closely on the map, IMHO the Mongols would have an easier time subjugating Japan had they also invaded via Sakhalin (Karafuto) & Hokkaido islands in a pincer movement, using the local Ainus as guides/auxiliaries- it would be interesting to explore how such an operation could have unfolded!
Afterwards, to ensure against revolts, relocate & disperse most of the population to the Asian mainland & incorporate the samurais into the army & navy units there. Reward the Ainus by giving them their lost lands & autonomy, while settling Japan with Korean & Chinese military colonists under the overall Mongol control. Open a ferry between the mainland & Karafuto, Hokkaido & Honshu,etc to connect Japan with Empire's vast road network. Had all of the above happened, the Kurils, Aleutians, Alaska, & N. America's Pacific coast (to say nothing of E. Siberia) would probably have been colonized by E. Asians over the next 200-300 years!