“Although many might be willing to admit that one’s duties toward one’s own take precedence over those toward mankind at large, it might well be asked why it is necessary to harm enemies, or why there need be enemies at all. The answer is twofold. There are unjust men who would destroy the good things and the good life of one’s own family or nation if one did not render them impotent. And, even though there were not men who are natively unjust, there is a scarcity of good things in the world. The good life of one group of men leaves other groups outside who would like, and may even be compelled, to take away the good things of the first group. To have a family or a city that is one’s own implies the distinction between insiders and outsiders; and the outsiders are potential enemies. Justice as helping friends and harming enemies is peculiarly a political definition of justice, and its dignity stands or falls with the dignity of political life. Every nation has wars and must defend itself, it can only do so if it has citizens who care for it and are willing to kill the citizens of other nations. If the distinction between friends and enemies, and the inclination to help the former and harm the latter, were obliterated from the heart and mind of man, political life would be impossible. This is the necessary political definition of justice, and it produces its specific kind of human nobility expressed in the virtue of the citizen.”
See also: The Concept of the Political by Carl Schmitt (1933).