I here focus in some depth on the causal relationships between nationalism and the two world wars of the twentieth century, asking two main questions: did nationalism cause these wars and did these wars intensify nationalism? Nationalism is generally defined as an ideology embodying the feeling of belonging to a group united by common history and a combination of ethnic/religious/racial/linguistic identity, which is identified with a given territory, and entitled to its own state. There is nothing inherently aggressive about nationalism, though it becomes more aggressive if one's own national identity is linked to hatred of others' national identities. Most scholars of nationalism would not claim that a sense of national identity is ever total in the sense of displacing all other identities, but they do tend to argue that, whether overt or latent, nationalism has dominated modern warfare. It is highly likely that a war between countries in the age of nation-states will have the effect of increasing the aggressive component of nationalism, but I am more interested in the reverse relation: does nationalism cause war? Clearly nationalism has to take rather aggressive forms if it is to do this.

This question can be first addressed by asking whether nationalism and war have tended to rise and fall together, in roughly the same time and place. There seems to be general agreement that nationalism first became widespread in Europe at some point in the eighteenth or nineteenth centuries, became generally more aggressive at the beginning of the twentieth century, and then spread out to the world in both mild and aggressive forms. So I initially ask whether this corresponds in any way to the incidence of wars in Europe and then the world in the modern period.