By 1919, had the idea of nationalism done more to unite people, or to divide them?

Introduction

The term nationalism refers to political principle, born in the late eighteenth century to legitimize the existence of a nation-state for each people (initially opposed to the monarchy, political system in France which was then appointed as Old Regime), which has emerged during the nineteenth century and early twentieth century. The historians do not describe nationalism, in general, as an ideology because it is little and poorly reasoned or justified by intellectuals, it is against this as an easily identified in the political and social life. ...