Of course, the time frame you mention is very long and so nationalism had a number of different impacts during this time. One way to describe the overall impact is to say that nationalism led to a remaking of the map of Europe and to a number of wars.
Nationalism had the effect of remaking the map of Europe as old countries broke apart and new countries came into existence. For example, the countries of Italy and Germany as we now know them came into existence because nationalist impulses led to the amalgamation of a number of smaller states into the countries we know today.
Nationalism also led to wars. Most notable among these was WWI. World War I was caused in part by the nationalist desire of various peoples to rule themselves.
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