When you don’t understand the meaning of words.
Nation is a word that is rarely understood. It is not the same thing as country or the state. Nation refers specifically to a group of people connected by blood. Usually, implied in that blood connection is shared language and religion.
The idea that people could band together along strictly ideological lines has never been believed in any point in history up until the last one hundred and fifty years. There’s a reason for that… it’s because the idea is stupid. People care first and foremost about their family, both nuclear and extended. In fact, even when people are taught to hate their families, which is one of the projects of modernity, it still can rarely be accomplished. That is the totality of nationalism and the controversy surrounding it: caring about one’s family more than other family groups.