The engineer trilogy, book 3 : The escapement
K. J. Parker2007

Condemned to death, the engineer Ziani Vaatzes escaped from Mezentia. His implacable determination to return home led him to contrive an intricate plan to bring down the Republic, a plan which require a war. He regrets the deaths. But he had no choice. Duke Valens dragged his people into the war to save the life of one woman. Then he killed her husband. Now, allied to the nomadic barbarians who have the manpower to defeat Mezentia, the Duke regrets the evil he's done. But he had no choice. Secretary Psellus never wanted to rule the Republic, or fight a desperate siege for its survival. Trying to make sense of how it all happened, he finds himself compelled to investigate the circumstances of Vaatzes' crime. He is terrified of what he might find out, but he has no choice. The machine has been built. All that remains is to set it in motion.