According to Wikipedia:

Toyotomi HideyoshiFebruary 2, 1536, or March 26, 1537 – September 18, 1598) was a Sengoku period daimyo who unified Japan. He succeeded his former liege lord, Oda Nobunaga, and brought an end to the Sengoku period. The period of his rule is often called the Momoyama period, after Hideyoshi's castle. He is noted for a number of cultural legacies, including the restriction that only members of the samurai class could bear arms.

(Wikipedia contributors, "Hideyoshi," accessed 2/16/08)

This reference first appears on page 191.
Robinson references Hideyoshi in order to use an real historical reference to begin Book 3 in Years of Rice and Salt. He is the Japanese Emporer that is fighting against China.The conflict between China and Japanese is used to set up the accidental journey to the new world. Hideyoshi is the emporer of Japan while at war with china.