The correct answer to this open question is the following.
The dictionary definition for the word internecine to fit this context is the following.
According to Merriam-Webster., the word Internecine means mutual destructive. Deadly.
So in this case, the context of the excerpt means that both sides, Red Republicans and Black imperialists were fighting each other in a war that only hurt them equally with no benefit at all. This term is used when a group of people or two factions are fighting each other with no real winners and so much harm.