Probably a result of having decades of wars and conflicts that weren't justified since then. The government breaks trust with the people over and over again that soon that is all we know, and some people start to question the past justifications. I personally think WWII was absolutely justified, but it doesn't surprise me that confidence is slipping across the general population. People just don't trust the government anymore.
"31 percent of the Americans surveyed, and 41 percent of millennials within that group, do not believe that 6 million Jews were killed during the Holocaust and think the real death toll is at least 2 million lower."
And let's be fair, there are a lot of really stupid things that the "average" American believes.
Only 81% of Americans say humans have evolved over time.
63% "Believe in God, absolutely certain"