Maria thanks a lot for posting a reply in response to my query, however, the statement in the question is "It (Nationalism) has contributed to the further division of state in the world but on the contrary leading to the unification of Western Europe".

In my view, the foundation of a nation-state is constituent upon three pillars; Sovereignty, territorial integrity, and equality of states. Sovereignty implies that a Government possesses virtually unlimited authority with respect to a state's internal affairs and foreign policy, however as seen during 19th and early 20th century, Nationalism changed the world map. Ottoman Empire changed shape, (Czars) Russian Empire broke down into Lithuania, Latvia, Estonia, Romania, Georgia, Armenia etc.. Germany on the other hand transformed from an oppressed state into an aggressor following Hitler's rise into power, so Nationalism did change the Westphalian State system, as the authority of the Government was challenged. In recent times, we have heard of terminologies such as "Black Nationalism" used frequently by Malcom X, or Greater Balochistan used by BLA, or even Tamils in SriLanka demanding a separate state for Tamil people, thus it would be conclusive to say that Nationalism has resulted in the division of the world to a certain extent.

About the point of Western Europe unification, in my opinion "British Nationalism" can fit well within to the dimensions of this question because I can recall this as the only instance since Napoleon Bonaparte, where Nationalism has helped in bonding four countries; England, Wales, Scotland and Northern Ireland.