Crisis is often very important in bringing about feelings of nationalism and, in particular, ultranationalism.
When crises hit, people often tend to pull back and seek refuge in things that feel familiar. They often blame the crises on things that are unfamiliar. This promotes ultranationalism. For example, the Great Depression in Germany certainly counted as a crisis given that the country was already on shaky ground due to WWI and the reparations it had to pay as a result. When the Depression hit, Germany was shaken badly. People came to blame the Jews and the communists for the problems and to cling to the ideas of Nazism for refuge.
Crises, then, can help to make people feel more nationalistic.