A more thorough explanation:

Definition: Internecine (in-tər-nee-sin or in-tər-nee-sIn or in-tər-nes-een) is an adjective that describes:

1. Deadly; characterized by mass slaughter.
2. Mutually deadly; destructive of both parties, for example, an internecine civil war.
3. Loosely, of or relating to conflict within a group, such as internecine faculty politics.

Examples:

• The internecine battle between the two rival gangs left dozens dead.
• The internecine conflict between the two political parties resulted in the downfall of the government.
• The internecine squabbles among the members of the board of directors led to the company's bankruptcy.

The examples illustrate the different ways in which the term internecine can be used. The first example shows how it can describe a situation of mass slaughter, while the second example demonstrates how it can refer to a mutually destructive conflict. The third example shows how it can be used more loosely to describe conflicts within a group.