The book "Imagined Communities: Reflections on the Origin and Spread of Nationalism" by notable political thinker Benedict Anderson is regarded as one of the most important works written about the sources of modern nationalism.
Key to Anderson's understanding of nationalism is the notion of "imagined community", the fact of nationalism being not something material or natural (as often thought) but rather something that exists in the connection between culture and psychology. The national community is defined by Anderson as "imagined" since its members do not personally know each other but yet they bear in their mind the thought of mutual connection. Anderson holds that any community larger than the traditional village is essentially an imagined community.
At the beginning of "Imagined Communities" Anderson argues that in order to understand nationalism we must look at the manner in which national identities have formed over time which can account for why they are so meaningful today. According to Anderson's thesis the development of printing press alongside the Protestant revolution in Europe brought about a large popular readership of non religious, non Latin texts. This had a key role in the development of languages that were mutual to large populations, languages that over through Latin is the dominant language of power.
These new languages, according to Anderson, enabled mutual unified forms of communication that were soon adopted by regimes, thus setting the ground for the appearance of a new form of imagined communities - nationalism. Since someone in the north of modern day France and someone in its south could converse easily, read the same books, same newspaper, abide by the same law, answer to the same courts and eventually serve in the same army against people whose language they cannot understand, they began to imagine themselves as brethren, being part of the same community which was and still is, imagined.