What wars are called internecine? Why did they weaken Greece?

The Greeks played one of the most important roles among all the peoples of antiquity for the development of civilization and science. Greek individual policies were more powerful than large countries, and when several policies were united, they could even defeat such a formidable enemy as Agemenid Iran – the largest and most strong country of those times. However, they constantly fought among themselves (the war is between representatives of one nation is called internecine), thereby weakened each other, and as a result they were conquered by their neighbors.