How does responsibility differ from accountability? Simple. Those who make the decisions are responsible in that they were elected for that purpose: They are responsible for making decisions or executing them. They may even take the blame for making bad ones. However, they never really pay the price for those decisions. They never actually suffer the consequences of them. More often than not, they are reelected so they can return to their seats of power and continue making bad decisions. Whose fault is that?
That being said, We the People electing poor representation to government is not a very satisfactory answer to the question: What is the greatest or costliest mistake in modern U.S. History? Ultimately, it is too broad. It means that We are both responsible and accountable for all mistakes. To determine which is the greatest or most costly mistake, We must narrow our vision to one specific mistake.