Seems like its a song expressing overall disillusionment in the prospect of ever improving the good of the masses. I agree with the above historical intrpretations (Camelot is particularly insightful). This would fit the contextual attitiude of young Americans during the late 60's, early 70's.

By the by, as I dogpiled for the lyrics, I came across this (Speaking of right-wing disillusionment):

http://www.freerepublic.com/forum/a36c901ff77a8.htm