I was told we didn't know about the concentration camps until after we were involved and went overseas, but then I hear "If we weren't fighting for democracy and righteousness, what were we fighting for anyway?"
Non-academic response/suggestion:
Well the most basic why would be an act of war (bombing of pearl harbor) was committed by a foreign nation. The US then declared war on Japan, and due to their alliance Germany then declared war on the US. Maybe the true question you seem to be after should be re-worded. You seem to be more curious about where American people drew motivation to fight/ continue WWII, as the reason for the US entering the war is relatively straight forward in a "series of events" / legal sense.