With regard to civic and ethnic nationalism, why would one argue that ethnic nationalism is the most devastating force in the world today?
Civic nationalism maintains that the nation should be composed of all those-regardless of race, color, creed, gender, language, or ethnicity- who subscribe to the nation's political creed. Ethnic nationalism, by contrast, holds that people's allegiance is to an ethnic group or nationality into which they have been born or assigned, not to a larger political entity encompassing many ethnic groups or nationalities.
We would argue that ethnic nationalism is worse because it is based on much more visceral and emotional factors and is less susceptible to reason.
Civic nationalism is less prone to extremism. It does not hold that some kinds of people are better than others. Instead, it prefers one set of political beliefs to another. People generally (with some exceptions for things like the Cold War) hold this sort of belief less intensely and with less inclination to hate those with other beliefs.
By contrast, ethnic nationalism is very much of an "us against them" sort of nationalism. It encourages people to be chauvinistically proud of their ethnicity, which necessarily requires them to dislike or disrespect other ethnic groups. It is much easier to arouse hatred on this sort of grounds as we have seen time and again in such places as Rwanda (Tutsi and Hutus) or China (Han Chinese and Mongols). Therefore, ethnic nationalism is the more destructive force.