From A Brief History of Khubilai Khan by Jonathan Clements, available now in the UK and in the US.

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The attempted invasion of Japan was, with hindsight, the moment when the Mongols' legendary invincibility was called into question for the first time – a sign that the tide of barbarian invasion had finally begun to ebb. The Mongols had experienced setbacks in the past, but had always, eventually, returned home with, at very least, the nominal submission of their enemies.

Logistically, the...