To say that socialism is the opposite, by definition, of nationlism is like saying that apples, by definition, are the opposite of oranges. It makes no sense. It makes even less sense than that, actually, because nationalism is not in any way exclusive of socialism, or vice versa. The current Chinese regime is both, for instance. Socialists usually do have a dream of spreading the revolution globaly, but there are mant ways to attempt this, some of which in no way exclude nationlism. The multi-cultural (really just uni-cultural) socialism of our own Left is equally as dangerous and potentially murderous as national socialism, BTW.
Pure socialism assumes a worldwide brotherhood of workers. Nations and nationalism are to be swept away. As Andy Card said a couple of days ago, the two concepts are at opposite ends of the ideological spectrum.
Walt