What is a word that describes a monolithic national identity?

I think you are looking for Chauvinism:

a form of extreme patriotism and nationalism, a fervent faith in national excellence and glory. It is an irrational belief in the superiority or dominance of one's own group or people, who are seen as strong and virtuous, while others are considered weak or unworthy.

A less strong but still apt term is Nationalism

is an idea and movement that promotes the interests of a particular nation (as in a group of people) especially with the aim of gaining and maintaining the nation's sovereignty (self-governance) over its homeland.

(Wikipedia)