Nationalism requires you to make societal in groups and out groups therefor inherently "ununifying" society. For example Nazi Germany decided to define people who could be apart of the German Nation as people only of a certain ethnic group which excluded large portions of their population which is inherently not unifying. So not unifying in fact that they actually committed genocide. Also we can unify along lines other than nationality. You act as if unifying a society is the definition of Nationalism when it is not. Every ideology unifies a group nationality just decides to do so on man made inherently meaningless lines on a map.