Final answer:
The word 'internecine,' in the context of the provided quotation, is best defined as 'a. destructive to both sides.' It refers to a conflict where all involved parties suffer significant damage or loss.
Step-by-step explanation:
The dictionary definition of the word internecine that best fits the context provided in the quotation is: a. destructive to both sides of a conflict. The context in the quotation, ". . . internecine war; the red republicans on the one hand, and the black imperialists on the other," describes a scenario where two opposing factions are engaged in a conflict that is likely to be harmful or destructive to both parties involved. This definition captures the essence of a conflict within which the combatants incur mutual destruction.
Internecine warfare is often characterized by bitter and brutal fighting, where the ultimate consequence is damaging to all groups involved, much like in the historical instances such as civil wars or revolutionary struggles that incur heavy losses on both sides.