I guess the Sons of the Pioneers got it right:

"See them tumbling down...."

People are surprised they were lied to because when war is coming people want to believe, have to believe, that their cause is Just and Right and Good. Nobody would approve sending troops if they thought otherwise. And people want to believe their leaders. Then, big surprise:

"We didn't know said the puzzled voter
Watchin' the President on TV
I guess we gotta drop those bombs
If we're gonna keep south Asia free
The President's such a peaceful man
I guess he's got some kind of plan..."