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

Internecine strife is the name given to wars occurring among representatives of one people or residents of one country. In almost all countries where internecine war breaks out, there is a crisis and weakening. It also happened with Greece. Paka Greek city-states were in allied relations and adhered to neutrality, they were strong, and even managed to stop the invasion of the powerful Persian Empire. And after a series of wars, first Persia began to interfere in the affairs of the policies, and then Macedonia captured many cities.