Ethnicity has threatened some states in Asia and Africa because these states are made up of multiple ethnic groups that do not get along well together. This is a legacy of colonialism.
For example, we see tensions in Iraq between Arabs and Kurds. In Nigeria there are tensions between various ethnic and religious groups. There was the infamous genocide in Rwanda that pitted the two major ethnic groups in that country against one another.
Because these states have been arbitrarily created with multiple ethnic groups within their boundaries, they often have trouble trying to maintain their cohesiveness and viability as states.