Napoleon discovered that nationality is what will win wars and enlarge land owned by those who connect themselves with that nation.
A New Nationalism
Many of the territories occupied by Napoleon during his Empire began to feel a new sense of nationalism. During the occupation, Napoleon destroyed and disallowed many nation's individual cultures, and the people of these nations greatly resented this. As a result, Napoleon's conquests spurred a new nationalism in the occupied nations, particularly in Germany and Italy, at a level that had never previously existed.
http://en.wikibooks.org/wiki/European_History/Napoleon_Bonaparte_and_the_Rise_of_Nationalism
Napoleon saw nationalism as indispensable to maintaining the loyalty of the French people to his regime. After all, it was the spirit of nationalism that had inspired its armies in a remarkable series of victories that had especially benefited Napoleon and allowed his rise to power. The trick was for Napoleon to build a personality cult around himself so that the French people would identify him with France itself and therefore make loyalty to him equivalent to loyalty to France. However, by identifying national loyalty with one man, Napoleon inadvertently weakened the inspirational force of nationalism and thus his own power.
http://www.flowofhistory.com/readings-flowcharts/the-early-modern-era/the-age-revolutions/fc106