According to Wikipedia:


Wanli Emperor (September 4, 1563 - August 18, 1620) was emperor of China (Ming dynasty) between 1572 and 1620. Born Zhu Yijun, he was the Longqing Emperor's son. His rule of 48 years would be the longest in the Ming dynasty and it witnessed the steady decline of the dynasty. Wanli also saw the arrival of the first Jesuit missionary in Beijing, Matteo Ricci. ... Hideyoshi's second invasion of Korea and China would be the one that occurred in the 25th year of the Wanli's reign.


(Wikipedia contributors, "Wanli Emperor," accessed 2/16/08)

This reference first appears on page 191.
Robinson use another real historical reference to begin Book 3 in Years of Rice and Salt. The Wanli Emperor sent Kliem to attack Japan. But Kliem fleet ended up going off course. This is used to set up the journey to the New World.