Jeff McMahan

“It is arguable […] that a further effect of our partiality for members of our own species is a tendency to decreased sensitivity to the lives and well-being of those sentient beings that are not members of our species.

One can discern an analogous phenomenon in the case of nationalism. It frequently happens that the sense of solidarity among the members of a nation motivates them to do for one another all that—and perhaps even more than—they are required to do by impartial considerations. But the powerful sense of collective identity within a nation is often achieved by contrasting an idealized conception of the national character with caricatures of other nations, whose members are regarded as less important or worthy or, in many cases, are dehumanized and despised as inferior or even odious. When nationalist solidarity is maintained. in this way—as it has been in recent years in such places as Yugoslavia and its former provinces—the result is often brutality and atrocity on an enormous scale. Thus, while nationalist sentiment may have beneficial effects within the nation, these are greatly outweighed from an impartial point of view by the dreadful effects that it has on relations between nations.”
― Jeff McMahan, Ethics of Killing: Problems at the Margins of Life