Nationalism is an ideology which affects social, political and economic political systems. It is based on the protection of sovereignty of the country and maintaining a national identity free of foreign intervention. It defends and promotes national interest over foreign relations.
Imperialism is an ideology or practice that promotes expansion and dominion over other countries, aquiring new territories and colonies, and exercising control over them.
For example, during World War II, the Japanese government promoted a nationalistic ideology amongst the people to create unity against foreign invasion and colonization. Fear was used to generate nationalism, because they acted in response to foreign threats. Nationalism affects Imperialism because it reinforces the idea of one country being better than the other. Fear of external intervention leads to the idea the "offence is the best defense": in order to protect our nation, we have to expand our territory. It can be said that nationalism becomes an excuse for imperialism.
