When Kublai Khan invaded Japan for the second time in 1281, he sent over 4,000 ships. He required the invasion be mounted in a year. As a result, new ships were hastily built and seemingly of shoddy quality. There were some sound and very advanced ocean going ships, but the balance was commandeered riverine vessels.
Let’s assume a few things…Kublai had a Chinese advisor who knew which side his bread was buttered on. This advisor strongly recommended that the invasion of Japan be delayed until proper ships could be gathered and built. That would probably have taken about 3 years. Kublai agreed.
As a result the devastating typhoon, the Kamikaze, was avoided. The preparations of the Kamakura Shogunate were extended over an additional two years, perhaps leading to fatigue and weariness. Kublai, having sent 2 additional emissaries who were beheaded has perhaps found additional emissaries with a death wish for a third or fourth mission and is imperially pissed.
What happens if the Mongol invasion is able to land on the Japanese mainland? The Japanese have still not been adapted to the Mongol concept of war as a team effort. They will learn quickly, but for now they are still struggling with it.
***OK, let’s also forgo the argument that the Chinese could not build Ocean going ships. Their naval architecture was far in advance of anyone else at the time. The premise here is that Kublai has built proper ships for the mission and not commandeered every river ferry on the Yangtze.