This is because nationalism is never a disease restricted solely to the working class, or the gullible. It infects us all, to varying degrees. It's the most seductive of viruses; it starts out so well, so beautifully, as a form of passion, a love for oneself and others who share your beliefs, your faith, or your ethnicity. It acts as a binding glue which creates identity where perhaps no identity existed before or, more likely, where it had been shattered into fragments by aggressors.
Slowly over time, the emphasis shifts from commonalities to difference, from who is gathered together to who represents a threat from without. And resentment takes the place of generosity, and the nation becomes a bastion to be defended against aliens, and a notion arises that perhaps one should attack them before they can attack you. Nationalism is a drug. A very pretty drug, with centuries of ardent rationalisations created in defense of its use. It creates euphoria, a sense of superiority, and a feeling of belonging to something bigger than yourself. Is that not the effect of a drug?