Hehe, there is no society on earth right now that never incorporated substantial numbers of outsiders. The relatively placid world that we live in today is in no way the historical norm. Hostile conditions are the norm. Ask the Palestinians, the Iraqi, the Iranians, the Panamanians, the Native Americans and Native Canadians and lots of people living elsewhere in Europe and the world (or anybody else for that matter, but for first or second generation immigrants it is obvious) to see how far back their roots go. In most cases you won't have to go much further than three or four generations (assuming you can trace them back at all) before you land abroad and in many cases in wildly different cultures due to wars, colonialism, trade, famine or natural disasters. Japan is due to its state as an island an exception, not the rule and even Japan is subject to the maxim that you use it or you lose it.