The Brexit debate is a useful illustration of what nationalism actually is. The British, at least some of them, are desperate to claw back their ability to fully govern themselves through their own institutions. This simple, elemental motive is at the root of nationalism. Now, nationalism can have other permutations—for example, the American nationalist tradition running through Hamilton and Lincoln implies a variety of other priorities and commitments. But Brexit is a pretty good test—if you sympathize with the British attempt to be a fully sovereign country again, you are a nationalist of some sort; if you side with the E.U., and believe that the nation is crimped, small-minded and unworthy entity compared to supra-national institutions, you most definitely are not.