You can thank our educational system (and educators) for that... If it wasn't a version of history that highlighted the very worst of what was perpetrated by the US than it didn't matter...
I once asked my A student daughter what she was learning in history class (sophomore year in high school) and she said they were studying WW2... I asked for details and she elaborated that they were being taught that it was all just a "political disagreement"... I asked if the minor detail of "good versus evil" was covered at all and the reply was a negative..., aside from the "fact" that dropping the A bomb was evil...
I took a much, Much greater interest in her history education from that point on, making a point of taking her on a history trip/vacation to D.C. (and surrounding environs, hitting nearly every important site along the way (from the French & Indian War [ Fort Neccessity & Braddock's Road] up through the Civil War [Harpers Ferry, Antietam and Gettysburg] and on thru WW2 [National Musuem of the Marine Corps, Arlington Cemetery, Air and Space Musuem Annex at Dulles]... By the end of that trip she actually knew a little something about American History... (and had much greater appreciation of her families contribution to it)... While I'm glad to have done it (become involved in her education) it's pretty damn sad that I had to worry about it to that extent...