I suspect it's a quite complex matter. It's probably a built-in human instinct to defend a territory. Humans are social animals, and when the population was small, you would know all the members of your group, so members of other groups would be easy to identify. But, at the same time, to exclude foreign groups would be a recipe for disaster- inbreeding would render the group very ill within a few generations. So you end up with loose coalitions of kin- groups, which interact in more-or-less ritualised ways.

As population grows, the probability of encroaching on each other's territory grows, and developing over the millennia we find situatiions like New Guinea, where there was no possibility of spreading out, so tribes (extended kin- groups)were in a state of permanent, ritualised war.

In modern society, there's no natural tribe- most people don't even know the occupants of next-door-but three except perhaps by sight. But with the instinct still in place, they have a need to categorise people into Fella-Him-Belong and the Stranger Man. It's understandable, if not defensible- the instinct is now pretty well useless, and almost anything that can be discerned as a difference can be used- language or accent, dress, hairstyle (especially facial), skin colour, habits, superstitions and customs.

No Welshmen in the city after dark, no kilts or bagpipes, no Jews in the golf club... there's nothing new about it. It makes an easy appeal for unscrupulous politicians, and the disaffected are always on the lookout for somebody to blame for their woes. It's no coincidence that the BNP has its greatest success in areas of declining traditional indutry like Stoke on Trent and East Derbyshire - people there have a lot to be resentful about. The actual culprits- government and global money jugglers- are too remote to be real to people of limited vision, and its all too easy for them to take out their grudges on visible targets.

In Britain, the real failure has been in government- particularly in the Labour Party, which has been happy to allow their voter base to blame illegal immigrants (code phrase for "wogs") and bogus asylum seekers (code word for "nig-nogs") for problems, while getting their own feet comfortably under the polished directors' tables.