Why would nationalism stoke tensions between citizens? The point of nationalism is to unite the society around the nation.

It seems to me that multiculturalism if anything leads to more tensions as it seeks to add more groups that don't have as much in common as the people already there.

Contemporary history suggests nationalism is almost always a reactionary movement. Reactionary movements are by definition in reaction to some other political entity. Thus the tension.

That's true (at least where there are large nationalist and globalist groups in conflict), but it seems to me a lack of nationalism may sometimes result in suppression of the expression of self-interested sentiments, especially when globalism does not decrease overall GDP but causes large changes in the distribution of income/wealth.