Oh jeez, why do things have to be all one way or another?
Could it be that nationalism and supranationalism can both sometimes be peaceful, or sometimes lead to conflict? That any group can have hostility with other groups, or cooperate successfully with them? Hmm?
Could it be that these things depend on specific circumstances, rather than some fundamental rule of the universe that determines one or the other is peaceful or hostile?
Could it be that European integration has been a major contributor to stability in Europe, AS WELL as American military might? Why are the two mutually exclusive? I think you will find they were complimentary.
Could it be that there are good things and bad things about European integration? That it results in neither a utopia nor a dystopia? Hmm?