American History in the second-half of the Nineteenth Century and early Twentieth Century

Which of these specific lessons do you believe to be the most important to teach a class about American History in the second-half of the Nineteenth Century and early Twentieth Century?

Imagine you are tasked with delivering a single lecture based upon only ONE of the following Video Lessons for this Unit:

Lesson 3: Moving to the City

Lesson 4: A Dream Deferred

Lesson 5: Labor's Struggles

Lesson 8: The Progressive Paradox

Lesson 9: The War to End All Wars

Which of these specific lessons do you believe to be the most important to teach a class about American History in the second-half of the Nineteenth Century and early Twentieth Century? Why do you believe this is the most critical information for students to learn?