Notes on Nationalism is a 1945 essay by George Orwell.
Despite it's name Orwell is addressing a more general concept which can apply not only to nationalities but to any groups of people - whether it be an ethnicity, a religion, a social class, a political ideology or as a purely negative notion such as antisemitism or anti-homosexual ideologies. While the examples Orwell gives are of a post-war flavour it's easy to see how they directly transfer to today's world.
Why am I writing about it? Well, not only is it a fantastic essay offering a view of the post-war intellectual atmosphere, it says something very deep and fundamental about human psychology that is as relevant today as it was in 1945. It is something I would highly recommend anyone to read whether their interest be in history, psychology or philosophy.
Two links where the essay can be found: