US history

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 7: The Question of Empire 

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? Why do you believe this is the most critical information for students to learn?

Please make your initial post at least two (2) paragraphs long of minimum 500 words and include specific historical details, processes, and/or people to support your arguments.