The second of three stories nominated from There Will Be War X.
A new, cutting-edge Japanese military ship, JS Takao, carrying a new, secret weapon they plan to test on the edges of the solar system, is just arriving near Titan when the American base there hails them. They're being approached by ships that are behaving very much like approaching pirates--or perhaps someone's military, attempting an undercover attack, after which that country's military could "restore order." Can the Japanese help?
They can, and do. None of the attackers are taken alive, but the bodies recovered are all Chinese, fit, and of military age. When Chinese military ships approach very shortly after, there's not any realistic opportunity to consult Washington and Tokyo; the Americans and Japanese just have to make their own best decisions, make sure they document everything they do and why, and ensure that the Chinese reveal their hostile intentions first.
What follows is a pretty well-done, nicely tense space battle, with the truth about the Japanese secret weapon emerging gradually. It's a nice touch that that weapon turns out to be very powerful, but not magic, even by Arthur Clarke's definition of magic. I wasn't completely satisfied with the ending, but that may be just my lack of real enthusiasm for this type of story.
I don't think it's Hugo-level good, but you won't regret the time spent reading it, either.
I received the There Will Be War X anthology as part of the 2016 Hugo Awards voters' packet.