It seems to me that idealism played a significant role in the Enlightenment, which affected European culture (France & the revolution and its influence on the foundation of America). And reading up on the Japanese administration's pre Hiroshima dealings made them seem very idealistic (preparing for a last stand beachfront against invasion, saying they would fight to extinction).
I know military announcements are almost certain to be idealistic for troop morale and citizen PR reasons, but the stereotypes of Japanese culture go much deeper.
Similarly, I suppose any revolution might be pointed at as idealistic but while that may be the surface reason to revolt, most people are far more practical and only revolt when conditions are clearly bad. Like CGP Grey's video Rules for Rulers, the powerful turn on the leader when they have a new lackey in mind.
I'm thinking of idealism as a force for changing what a culture values, like the shift that occurred in the Enlightenment. Or as a force which drives people to start a nation, founded on different principles (blurring into politics here). Was the reason the Enlightenment caught on because it was made clear (by example maybe) that the new values were practically better and that the old ones wouldn't suffice anymore? Was it a pragmatic result from the increasing wealth of the everyman?
Cultures are also changing constantly; pre Hiroshima Japan was still modernizing from when the US forced them to open their borders for trading. It is likely impossible to know how a culture formed or how idealism played a role but are there cultures (and how old are they) where a greater proportion of people are idealistic? What influences people to be or become idealists? Is it related to fanaticism?
What culture do you think has been founded on idealism?