The world of The Poppy War is a very lightly reskinned China. I'm not particularly knowledgeable about Chinese history, but even so I was able to recognize multiple anecdotes drawn almost verbatim from Sun Tzu or the Romance of the Three Kingdoms. The events that occur over the course of the book mirror the Sino-Japanese war and the Rape of Nanjing.
I was going to write that I enjoyed the book despite this reliance on history, but that's not quite correct. It would be more correct to say that this reliance on history did not detract greatly from my enjoyment. Like The Traitor Baru Cormorant, this book is meant to grip you, to pull you along, but not necessarily to be enjoyed by you, at least through many sections.
At the end of The Traitor Baru Cormorant, the titular protagonist has betrayed almost all of her friends in hopes of freeing her home island. By the end of The Poppy War the protagonist Runin has committed genocide in revenge for the in-universe Rape of Nanjing and other atrocities. Neither of these are particularly endearing histories, although both authors make sure you know precisely why the characters act as they act. You cannot help but be sympathetic to both. But in the end I have to think Baru is the better protagonist than Runin; although many will disagree with her coldly utilitarian moral center, Baru nevertheless has a moral center in a way Runin does not.
This isn't a criticism of The Poppy War, however. War is hell; war breaks down whatever moral center you have; this is the central lesson of the book. It isn't pleasant to look at, but it isn't meant to be pleasant, because the reality is not pleasant.
(Sidenote: This book is exactly like The Name of the Wind in having a kind-of-crazy teacher on faculty in a school, who nevertheless turns out to be very knowledgeable, and important on our protagonists journey to Deep Magic. I'd like to see a book where there's a kind-of-crazy teacher on faculty in a school, and then our protagonist tries studying with them, and it turns out to be a complete waste of time. A little more true to life.)
‘Total victory,’ she said. ‘It’s what you want, isn’t it?’
‘What I want?’ The Phoenix sounded amused. 'The gods do not want anything. The gods merely exist. We cannot help what we are; we are pure essence, pure element. You humans inflict everything on yourselves, and then blame us afterward. Every calamity has been man-made. We do not force you to do anything. We have only ever helped.`
'This is my destiny,' Rin said with conviction. I’m the last Speerly. I have to do this. It is written.'
“Nothing is written,” said the Phoenix. “You humans always think you’re destined for things, for tragedy or for greatness. Destiny is a myth. Destiny is the only myth. The gods choose nothing. You chose. You chose to take the exam. You chose to come to Sinegard. You chose to pledge Lore, you chose to study the paths of the gods, and you chose to follow your commander’s demands over your master’s warnings. At every critical juncture you were given an option; you were given a way out. Yet you picked precisely the roads that led you here. You are at this temple, kneeling before me, only because you wanted to be. And you know that should you give the command, I will call something terrible. I will wreak a disaster to destroy the island of Mugen completely, as thoroughly as Speer was destroyed.”