such issues have been matter of politics, philosophy and art for couple of centuries. Relatively young ideology, which blossomed out of obvious need. In past loyalty been for leader or city, or other smaller concept to grasp. In times of French Revolution or American Revolution, concept of nationalism finally born.
One can see clear logic of american white men being very annoyed under the fist of british empire. Willing to govern themselves, not by distant authority that merely rips off the wealth. Nation is something what was build, not something that just "was there". Of course it's in a way very selfish method, which is in conflict of some other groups of people (Birth of USA perhaps easiest example).
It is purely my own assumption, but this seems key element of Brethren. Rejecting the distant authority, who's interest is merely leech and abuse. It shows very simple, but obvious traditional logic of how pattern goes.
Oneself, his family and friends, the close surroundings of people of similar interests. Local possibilities of living and surviving. Possibilities make decisions what affect your life. To put your earnings to be used for causes you believe are right. To see collective matters used for collective benefit, and not always sold off to foreign abuser to even rip off profit from simplest utilities. Meanwhile it addresses selected social problems in general.
It's a romantic vision of course. Most people can't feel anything common with the neighbor or other people in general. But yet, still most tend to prefer to see even anonymous country mates living next door than group of gypsies or whatever is the usual local nuisance which stands out.
Due living in Finland, I have no need to really take baggage of imperialist policies or such. My view on usefulness of nationalism can't be really argued with slaughter of native americans or whatever.
