Nationalism is one basis for most wars. A more important basis, however, is the search for profit and control of material resources...such as arable farmland, oil, water, timber, etc...

Nationalism is an arbitrary cultural notion that results in the drawing of fictional borderlines on the natural World and the raising of competitive armies and systems within those borderlines. It is a handy excuse with which to clothe the search for material gains and political hegemony.

Another basis of some wars is differing ideas about religion and other forms of cultural heritage.

Another is racial or tribal identity or even language!

Another is competition between different social theories, such as capitalism and communism.

All of the above divisions are foolish and essentially arbitrary. We could do well to celebrate our diversity, rather than being frightened by it.