None of the above. I mean imperialism has a lot to do with it but I attribute nationalism to a certain concept known as hegemony. Ruling classes, especially under capitalism, want a certain conformity of thought. It is easier to sell to an entire market of people who all crave the same things that simbolize the same dreams (ie, the american dream). Look at what people built for themselves as far as furniture goes 200 years ago compared to the cookie cutter repitition of today's Ikea culture. The things that people consumed back then had a certain degree of variance and uniqueness. Today everything is cookie cut to appeal to a single mass market. In order to create that market we require a unified belief that this one type of product is all that is good and holy. The same logic applies when the nation-state needs to go to war on behalf of capital (ie, today's war on Iraq). We, as Americans, are encouraged to support this war to protect what we have invented, democracy and freedom. Americans are taught to believe they invented these ideals, and for the most part this type of nationalist conditioning works. If it didn't we would all be fighting for a new kind of system. In my opinion its only a matter of time, peoples's lives aren't getting any better and its becoming harder for the ruling elites to justify why.