The nationalist
Broadly speaking, a nationalist is a person who finds identity in their citizenship to a nation, but it isn't that simple. Anderson says that more specifically, a nationalist is someone who has constructed a narrative out of their citizenship. Their citizenship makes them feel attached to the history of the nation with emotional attachment, so that nationalists end up with passionate emotional attachments to their own citizenship, for better or worse.
The monarch
Another major change in the flow of history came when America walked away from the monarchal system of Britain, a decision that has had its ripples throughout the Western world. The change was not just a big government change. Anderson says that without a monarch to fix one's political identity upon, the space psychologically suited for such an attachment is left grasping at an abstract "government" which doesn't have a specific figurehead, because the president is elected for a term. This leads to nationalism, says Anderson.
The community
In the past, the community in which one belonged could easily occupy part of their psychological relationship to self, or their identity. In the distant past, no person was dispensable in a community—everyone knew everyone else's name. Now, in the present, community has become abstracted. That abstract kind of community, in Anderson's opinion, is just begging to be reinterpreted, to be fixed down, and so people make concrete opinions about identity rooted in other things, like national citizenship.