One of the ideas which has, perhaps more than any other, led to war and suffering in the modern age, is the idea that countries should have clear ethnic/national identities which define their borders. This is something that we in the the US, which has been heavily defined by immigration and thus lacks a distinct ethnic national identity, but it is something which comes into stark relief when we look at conflicts in other parts of the world.
Of these, the one that gets the most press is, of course, the conflict over the Holy Land, where different factions insist that the same ground should belong to either a Jewish State or a Palestinian State. This leads to strife because obviously if the state in a given area is specifically intended to belong to one ethnic or cultural group, then members of other groups must either leave or see themselves as living in someone else's country.
This would work very well if various ethnic groups had spontaneously generated from the soil of different regions, but this is not the case. (After all, if you trace it back far enough, we're all Africans.) Recorded history is one long story of migrations, conquests and assimilations. And so in most cases where two different ethnic or cultural groups claim the same territory, they are both right, they are just pointing to different periods in history. This is true not only when we look into the past, but also, unless we assume an end to history, as we look into the future.
Thus, for instance, those who oppose the state of Israel often point out that Jews were a minority in the Holy Land prior to 1948, as if the question of the ethnic make-up of the region at that particular moment in history should settle the question of who controls the land in perpetuity. But, of course, the history of nearly any region is the history various cultural and ethnic groups moving in, gaining dominance, and fading in their turn. England, for instance, was invaded by Romans, by the Angles, Jutes and Saxons, by the Danes, by the Normans. Greece and Asia Minor have a long history of back-and-forth stretching back into mythology with the Trojan War, and ending with the mutual expulsion by modern Greece and Turkey of each other's ethnic groups in the '20s -- ending at least 2800 years of Aegean polyglot history with the stroke of a modern border-drawing pen.
This idea that countries should be dominated by a single ethnic or cultural group resulted in a number of mass expulsions during the 20th century, of which the flight of Palestinians from Israel during the '48 was is certainly not the largest. The partition of India and Pakistan resulted in the displacement of over 12 million people, and estimates for the death toll range from range from "only" several hundred thousand up to a million. After World War II, East Prussia completely ceased to exist, ending it's 800 year history. A total of 12 million ethnic Germans were deported from other countries into Germany after World War II, some from areas in which they had lived for up to a millennium, in order to assure that the new Europe would be ethnically homogeneous enough to avoid future wars, with roughly half a million dying in the process. The history of the Balkans in the 20th century is practically one long history of attempts at ethnic cleansing based on this concept of ethnic and cultural nationalism.
The mildly depressing thing is that these actions are generally considered successful when massive dislocations achieve their objective of ethnically and culturally united countries (as with the largest dislocations, those in Europe and between India and Pakistan) and only remain well recognized tragedies when the region remains unstable, as with the Holy Land and the Balkans.
So long as this idea that each cultural and ethnic group deserves its own country, and that that country must be on a particular piece of ancestral ground remains dominant, we can continue to expect wars and dislocations to result from it. It is an idea which is at odds with history. Attempts by international organizations to broker these disputes in a sense only give them more legitimacy, with many international organizations now endorsing two incompatible ideas: That on the one hand groups with "national identities" deserve to have their own countries and to do so on their ancestral lands, and that on the other hand that people have a human right to immigrate where they want.
This is not to say that conflict could be avoided by dropping this idea of nationalism. Conflicts have always resulted as cultural and ethnic groups have expanded their territory or migrated, and people who find themselves pushed out of or out numbered in areas in which their ancestors have lived for generations have always resented it and sought to reverse the process. But the idea that people have an absolute right to expect expansions and migrations not to take place, and that international organizations will step in to stop such events for occurring or reverse them, does nothing but grant further length and strength to that natural conflict.