In ethno-nationalist mythology, the ethnic group has existed for millennia, and has always yearned for a country of its own: this is the "primordialist" theory of ethnicity. The fact that people believe their ethnic groups to be primordial does not, however, mean that they are. Ethnic nationalism is a modern ideology which, for most of the eastern half of Europe, has been current for little over a century.
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NECESSARY CONDITIONS FOR ETHNIC WAR
Precisely when, then, does ethnic war occur? The key necessary conditions are:
Myths justifying ethnic hostility
 people respond to ethnic symbols and mobilize for war only if a widely known and accepted ethnic myth-symbol complex justifies hostility to the other group. The myths justify hostility if they identify a territory as the group's homeland which must be defended and dominated politically and define a mythical enemy with which the other group can be identified. …
Ethnic fears
A fundamental factor causing ethnic conflicts to escalate to war is that first one side, then eventually both sides, come to fear that the existence of their group is at stake. Such extreme fears justify hostile attitudes toward the other group and extreme measures in self-defense, including demands for political dominance. …
The source of such fear is typically the group's myth-svmbol complex, portraying the in-group as peculiarly under threat or peculiarly victimized.
In these cases, the more the group's historians emphasize the group's past victimization, the more credible are the emotional charges of genocide that arouse gut-level fears and the more appealing are hate-filled cries for vengeance….
Once ethnic fears become prevalent among the members of any ethnic group, for whatever reason, they justify and motivate a resort to violence in self-defense.