Nationalism influenced Adolf Hitler's rise to chancellor of Germany and he promised all of the German people economic reform and a chance to get back at the west. This was inspired by Mussolini's Fascist imitation of Rome. Japan also joined in, thinking themselves to be better racially superior to everyone, and they also wanted land and more natural resources. This sparked WW2, where all of the Axis powers committed atrocity after atrocity, and the wake of this brand of dangerous nationalism left millions dead.
Please note that no side was innocent in WW2, all sides committed atrocities.
A social contract often relates to the legitimacy of the authority someone has over the individual. The relation between legal and natural rights also is a topic of social contract.
