Since the war began, the nobles of Reinfield City have been earning a lot through the importation of weapons and armor. They have also earned a lot from the importation of food and the exportation of slaves.
As greedy as they are, they are content with sitting back and reaping large heaps of profits without endangering their lives. So they were determined not to participate in the war at all costs.
Even though they didn't want to fight the dukes, they still drew the line at participating directly in the war. They made it clear that they were more willing to fight the nobles and gang up on them than to fight the church of justice.
They threatened the other nobles, but it wasn't a rash thing to do. After all, if they are going to die by fighting the church of justice, they would rather die fighting the nobles.