According to Woodrow Wilson, things stand exactly so; "A nation which does not remember what it was yesterday does not know what it is today. We are trying to do a futile thing if we do not know where we came from or what we have been about." Woodrow Wilson - which means we are generally doomed, due to the prevailing popular opinion that "history is bunk"- as says the (in?)famous Henry Ford- among students. Ignorance regarding the subject most probably stems from such (lame) reasons as "well, if I major in history, I won't be able to find a job, except maybe teaching."
Ignorance of any subject is not a particularly promising recipe for success via future changes.
To actually answer your question, however...
To begin with, one must consider the concep of "free will" and whether or not it exists; if not, the we remain marooned in the Calvinist doctribnes of predestination (God already knows how your whole life will unravel, and he determines everything that will happen- you have no role in your future)- but if it dies, then history- complete with World War II and American Imperialism- has been the consequence of purely human choices.
Essentially, human beings learn by "doing"; a project may seem like a "good idea'" until just before it is implemented- look at the Bolshevic Revolution in Russian history, for instance; the beginning of the domination of Marxism (and the eventual murder of millions by the few in order to propogate "the idea" of economic equality SEEMED justified to the participants- "the ends justify the means," right?- but is now universally condemned as one of the bloodiest events in history.
Yet it was far from the "first time" something comparable had occured- people have been telling "white lies" for generations; (Hitler just happened to be the first to tragically exploit the idea that "tell people a lie often enough, and hey will come to believe it"); honestly, ignorance of history (perhaps in combination of widespread apathy/and/or lack of desire to use/lack of possession of/ critical analytical hinking skills (dependng on how cynical one wants to be) causes this phenomenon.
Hope that helps, and Good Luck!
THE AMERICAN PAGEANT - Kennedy; an A in both A.P. European, A.P. U.S. History