Er, spoilers about basic background of this author and poem. I'm sorry, I couldn't help myself... I got to searching and then...
So I don’t have the book yet but I did a quick google search about 天智天皇 (Emperor Tenchi/Tenji) and it was wild so I had to share. The Soga clan was essentially controlling the imperial family at the time (644/645) so Emperor Tenji (then Prince Naka-no-oue) conspired to kill the son of the Soga lord, and then when the assassins hesitated he did it himself (he was ~19 at the time). The Soga son lived long enough to plead his innocence in front of the Empress Kougyoku, but while he was doing so the guards finally rushed him and killed him. The Empress was rattled by watching the murder so she abdicated to her brother, who ruled for a while before dying, then she came back to rule again until she died, passing it finally to the prince. He then did a bunch of legal government stuff that made everything more central and powerful, including introducing the first Japanese legal code (according to records). He also invaded Korea once and failed, which is pretty standard.
Apparently the poem was written while he was traveling and there was a sudden rainstorm. He took shelter in a shack in a rice paddy and felt bad about how his subjects had to deal with these conditions every day. I think the last little bit could be read as either dew or tears, so that’s supposed to convey his sympathy.
I really really love history, especially that of Japan, so I’m looking forward to this! 