Baikoku Kikan

Briefly about Baikoku Kikan manga:

In the early 20th century, a small republic found itself stuck between a kingdom and an empire – both of whom wanted to manipulate the republic to suit their own needs. 

In order to protect itself from both sides of the conflict, the republic created a small group of soldiers whose job it is to suppress revolution and dissent in any way possible. This is the story of the Opera House.