Nationalism has been defined as follows:

"loyalty and devotion to a nation; especially : a sense of national  consciousness exalting one nation above all others and placing primary emphasis on promotion of its culture and interests as opposed to those of other nations or supranational groups."

The author of your quotation contends that nationalism tends to lead to war.  There are certainly many examples of this in history.  The Nazis, for example, believed that the German nation was superior to all others.  Thus, they embarked on a campaign of conquering their neighbors (such as Poland, Russia, France) and slaughtering Jews. 

The author states that nationionalism often leads to dissension and distress.  Dissension means division and disagreement between different people or groups.  Distress means suffering or discomfort.