How to trim this thesis to make it more concise, but still keep important details?
"In 1877, with the start of the second industrial revolution and the beginning of massive immigration of people from around the world, the United States entered a period of major political, social, and economic change after the Reconstruction Era. With these changes in American society came problems, and with these problems came attempts at reform. Politically, the control that corrupt political parties and bosses had once held over the American political system was successfully curbed with such innovations as the direct primary, the recall, and the referendum. Socially, the oppression of women and immigrants was somewhat suppressed by the women’s suffrage movement and the creation of settlement houses--though attempts at African American reforms were set back with the Supreme Court’s rulings in the civil rights cases of 1883. And economically, the overbearing power of trusts and their monopolies that had been present since the beginnings of industrialization and urbanization in America was broken by such decrees as the Sherman Anti-trust Act and the Square Deal. These reforms, both successful and unsuccessful, attempted to make the United States a more democratic, peaceful, and prosperous place to live for generations to come."
It covers all topics that my teacher had asked, but I think it's too long to be considered a thesis. It's more the length of a whole first paragraph...
I agree with you completely. It is too long to be a thesis. What you need to remember is that a thesis does not always need to state every topic you are going to cover in a paper. Your subject appears to be reform, its historical underpinnings, and its effect as a foundation for democracy and prosperity in the United States. A sentence that includes those three ideas should make a good thesis statement.
Sometimes, in a brief paper, the best thesis statement is one that carries the meaning of "This is true because of X, Y, and Z." Then one uses X, Y, and Z to write body paragraphs. However, for a lengthy history paper, which this is likely to be, a thesis statement that covered every single stop on the path to your conclusion would be too long for the reader to even remember by the time he or she got to the end.
It also seems to me that much of what you have now is really content that could be developed into several body paragraphs that provide the reader with the historical context necessary to understand how reform promoted democracy and prosperity.
History is complex, and whatever stance you want to have on it needs to be stated in a way that lets your reader know the path you will take to substantiate that stance, but it must be one that does not include every detail of the journey.