Nationalism has indeed promoted unification in Europe but division in the rest of the world. I won't define nationalism because that has already been cut/pasted above but I'll answer the rest of the question.

Most 3rd World countries were patched up by their colonial overlords. For example, the Hutu and Tutsi people in Africa have been chopped up and divided into five different states, in some the former comprise the majority and in others, the latter. Similarly, the Kurds have been divided into 4 different states, the Baloch into three, Pashtuns into two, etc, etc. These are all the doings of indifferent colonial masters.

In Europe, different nationalities (especially the Germans, Italians and Poles) were divided before the advent of nationalism (18th century). Nationalism brought together fragmented duchies into unified states such as Germany, Italy, Poland, etc. The nationalities of Europe took their destinies into their own hand.

So in Europe, nationalism stitched together divided nationalities, however, in the Third World, since the boundaries of the states were decided by colonial masters and not the native inhabitants, nationalities have been split up into different states and many of them want unified states of their own.

In the end, you can write a small passage about the two-nation theory and how all Muslims are one nation and what not. It'll please the examiner and earn you some valuable marks.

__________________
He conquers who endures.