What major event marked the start of Period 6 in U.S. history?

The end of the Civil War in 1865 is widely recognized as the event that marks the start of Period 6 in U.S. history. This period, which spans from the end of the Civil War through the beginnings of the Progressive Era in the early 20th century, is characterized by significant societal, political, and economic changes in the United States. The conclusion of the Civil War led to the Reconstruction Era, during which the nation grappled with the challenges of reintegrating the Southern states, addressing the status of formerly enslaved individuals, and redefining civil rights and liberties.

In contrast to the other options, the beginning of World War I occurred in 1914 and represents a later phase in U.S. history. The signing of the Declaration of Independence in 1776 and the establishment of the Constitution in 1787 are foundational events earlier in U.S. history, well before the Civil War and not related to the developments specific to Period 6. Thus, the end of the Civil War accurately captures the seismic shift in American society and politics that initiated the context for the issues and transformations that defined this pivotal period.