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IOTL the invasions failed due to storm. https://en.wikipedia.org/wiki/Mongol_invasions_of_Japan
There certainly were other logistical problems in hte planning and execution of the invasion itself, but still: what if the storm never happened that day? What if the weather was just fine?
Would the Mongols be able to make a landing? How far can they go into the coast? Will they be annihilated? Can they successfully invade Japan to the fullest? What will such an invasion look like?