A thousand years ago, most visitors to Japan would have arrived by ship at Hakata Bay, the one and only authorized gateway to Japan. Hakata was the location of the Korokan, an official guest-house for foreign visitors that is currently yielding its secrets to the spades of Japanese archaeologists. Nearby was Dazaifu, the imperial capital of western Japan, surrounded by mountain fortresses and defended by an army of border guards. Over the ages, Hakata was a staging ground for Japanese troops on their way to Korea and ground zero for foreign invasions of Japan. Through the port passed a...