Japan, AD 248
(A story set in Japan before the emergence of samurai? Oh, Issui Ogawa. That explains it.)
Miyo, the oracular Princess Himiko, has left the palace to walk through the countryside with her bodyguard Kan. They climb Mount Shiki and spy the distant harbor of Suminoe. Kan wonders if any of the ships they see might be from Wei, or Kentak, or Roma.
(Roma? Did the 3rd Century Japanese know about Rome?)
Miyo answers that the distance makes it unlikely -- the embassy she sent to Roma had lost half its ships on the roundtrip voyage.
(Diplomatic contact between Japan and Rome? That doesn't sound right at all.)
But sea trade has been improving, and it's only a matter of decades before permanent trade routes can be established. Already Japan has had contact with the red-skinned men of Kentak beyond the Eastern Ocean.
(Wait what?)
The men of Kentak and Roma had been eager to exchange laws and discover that Japan, like all lands they know, follow the Law of the Messenger, an ancient commandment for all people to cooperate with their neighbors to ward of the Disaster that must eventually come.
Suddenly a mononoke, a giant insect-like monster appears and tries to kill Miyo. Kan defends her, but he's no match for the beast. Then a mysterious figure appears and slays the mononoke. The man introduces himself as O, a messenger from the future, and warns Miyo that this mononoke was just the vanguard of an army that's gathering beyond her borders.
O, we soon learn, is an android from the year 2598. The mononoke, or ETs, have wiped out all life in the inner-solar system, and humanity has retreated to the outer system and extra-solar colonies. The war had stalemated, with what little momentum remained on the side of humanity, so the ETs constructed time machines to take the war into the past. Humans respond by dispatching an army of androids to the past to defend the timeline.
All fairly standard stuff. But the book is much better thought-out than most time-war stories I've read. For one thing, neither side mucks about with subtelty -- no one bothers with covert-ops to kill great leaders before they're born, or to wreck some important historical event. In fact, the Messengers have totally written off their original timeline and only wish to establish a victorious future. When they emerge in a past era, they immediately contact the powers that be, tell them the situation, and ask for help. Unfortunately this doesn't always help, and many of the new timelines fall to the ETs. And even if the Messengers do emerge victorious in one era, the ETs can just travel downwhen a few more centuries and start over.
As both sides move further into the past, they deplete their supplies. The ETs have to rely upon what they can build in each time, while the Messengers bootstrap local cultures to a level that can stand against the enemy. By the time both sides reach the 3rd Century ... well, things are pretty grim for both sides.
However, no matter how bad the situation gets, the book itself remains optimistic. Our Heroes may be fighting against a massive zerg rush with their backs literally to the sea, but the tone never flips to "Doomed, doomed, doomidy-doomed" mode. Just as in Tolkien, you know there's a eucatastrophe waiting to happen. When it finally comes, it borders on a deus ex machina, even though it follows logically from the rules laid out for time travel.
One thing I dislike about much SF is the way protagonists always have a post-Enlightenment mindset no matter what sort of culture they're from. Ogawa avoids this nicely, having Miyo be more alien than O. At one point, O describes the American Civil War to her and Kan, and they both respond in horror at the cruelty of the North for wanting to free slaves (they believe slaves would die without masters). Although Miyo's a strong female character, she is in no way a feminist in the way Robert Jordan's or George R. R. Martin's women are. She dislikes her position of mystic royalty, for which she was selected Lama-like, but she doesn't whine about it the way most Western heroes in the Campbellian style do. Instead of avoiding the Call to Adventure, she shoulders the responsibility because it's her responsibility.
O, for his part, is more than human without any of the pinnochioisms usually found in such characters in Western science fiction. He's not the sort to ask, "What is this love which you speak of?" He does have a quest for meaning in his life, but it's an entirely human one, not much different from what Mandella goes through in The Forever War.
The ETs, however, get no development whatsoever. They're nothing more than your typical bug horde, with no signs of reasoning despite their obvious technological prowess. We eventually discover that they were created by an alien race to wipe out humanity for reasons that would make the Minbari say, "Dude, that's screwed up."
The book is a mere 200 pages but packs more in than a thousand page doorstopper. One subplot of the book involves a Messenger who's composing a novel about caterpillars defending a tree from crabs that want to prune it. This allegory of the war, even half finished, is said to be longer than the Mahabhrata. We're given ten pages about timelines that Harry Turtledove could turn into a ten book series, and glimpses of dozen more equally epic. But Ogawa restrains himself to keep the story on track.