The authors conclude that americans who spilled their blood for the union emancipation and abraham lincoln who had promised a new birth of freedom instead got a bitter dose of corruption and political stalemate in the gilded age how can you account for this change over time

They realized that nothing really changed much. The freed African-Americans still had almost no political rights and the system was controlled mostly by corrupt politicians who came from the north to become rich. The regular people still had huge farms and turned the former slave system into a system of sharecropping which was in many ways similar to slavery. The freedom was very superficial and in reality people still lived as bad as before the war.

Marshall plan is the answer.