Dont forget your background sentences.  Background information provides your reader the context needed to understand the overall theme.  Start with a hook to get your reader interested, address the historical period you will be discussing, and provide needed definitions.  You only need approximately 3-4 sentences.
Your thesis statement should be the last sentence in your introduction.  Your thesis statement could be something similar to this, Imperialism, Progressivism, and the African-American struggle for equality during the late 19th and early 20th century did more to damage the concept that all men are created equal than it did to further it or, Imperialism, Progressivism, and the African-American struggle for equality during the late 19th and early 20th century (with notable exceptions) moved America closer to the ideal that all men are created equal than it had ever been before.
Then, in your 3 body paragraphs, discuss each of the topics:  Imperialism, Progressivism, and the African-American struggle for equality making sure to connect the information on each topic back to the thesis statement (your argument).
Your conclusion should restate your thesis in a fresh and interesting way and wrap up your argument.  This should be no fewer than 4 sentences long.