The events that accelerated the rise of aggresive nationalism in Europe after Wolrd War I were: the Great Depression in the United States, which triggered a worldwide depression marked by inflation and widespread poverty, as well as competition among European powers to accelerate their economies, which led to large-scale industrialization.
Nationalism is a political, social, and economic ideology and movement which seeks to promote the interests of a nation, aiming at gaining and keeping the nation's sovereignty (self-governance) over its homeland.