Populist nationalism is the contemporary right-wing political movement which is increasing in power in Europe and is represented in the United States by Donald J. Trump. It was foreshadowed by fascism and German national socialism. The movement rejects internationalism and embraces nationalism. Promising jobs and prosperity, populist nationalism appeals to sections of the working class, particularly those of the dominant ethnic component of the population.
Fascism, particularly Nazism, has a history of aggressive militarism which risks war. Much of this risk rises from intervention on behalf of national minorities in neighboring countries such as Sudetenland Germans in the case of the Nazis, or Russians in Crimea in the case of Russia. Devotion to the native population can, and has, included murderous racism.
There is no actual difference between populist nationalism and successful imperialist conquest as practiced by Britain or the United States during the Nineteenth Century, but, as winners write history, fascism has the connotation of nationalism as practiced by a weaker contender.