Once Japan became imperialist, a wave of nationalism swept through the country.
What is nationalism?

Identification with one's own nation and support for its interests, especially to the exclusion or detriment of the interests of other nations.

Example:

Any situation in which a nation comes together for a specific cause or in reaction to a significant event. The Battle of New Orleans in which Americans united at the conclusion of the American Revolution. The waving of flags and passionate anthem singing.