That's not true at all. Europe has always had conflict between countries and often even within countries between different groups. Ethnicity is literally any common group identity that is not universal. So if a group of people who live in the mountains start dressing a certain way, and worshiping a certain way, that becomes an ethnicity. There are literally countless ethnic groups. And it's not a meaningless term because those ethnic differences are very important to people. It identifies your tribe/community. It's usually a far more important identity to people than their nationality. Ethnic conflict has always been rampant, just like sibling rivalry. Ethnic groups are far more likely to fight with other local ethnic groups because they are in competition for scarce local resources.