Please note that answers on this site cannot be more than about 500 words so I can only devote about 160 words to each of these chapters. This means the summaries cannot be extremely detailed.
In Chapter 10, Zinn focuses on the idea that the elites of America before and after the Civil War used the middle classes as allies to help them keep the lower classes down. During much of the 1800s (particularly from about 1830 on), there was a wide gap between rich and poor in America. Zinn argues that it would have been natural for the poor to rise up against the rich. He mentions the Anti-Renter movement and Dorr’s Rebellion as two instances in which the poor did rise up. He then asks how it was that such rebellions did not happen more often. He says that this was because the elites enlisted the support of the middle classes to help them keep the poor subjugated. Because of this, the elites were able to get the government to help suppress things like the labor movement that arose in the late 1800s. Zinn ends the chapter by discussing at length the violent labor unrest of the 1870s which, he says, illustrates his main point in this chapter.
In Chapter 11, Zinn looks at the robber barons during the time after the end of the railroad strikes of 1877. He details the ways in which those men rose to wealth and power through unsavory means. He talks about how
in industry after industry (there were) shrewd, efficient businessmen building empires, choking out competition, maintaining high prices, keeping wages low, using government subsidies.
These robber barons were supported by the government, but they were, Zinn says, opposed by the common people. To illustrate how the common people opposed the robber barons, Zinn discusses a variety of instances of labor unrest during the late 1800s.
In Chapter 12, Zinn is interested in the overseas imperialism that became important in the US in the 1890s. Zinn argues that the imperialism was caused in part by a desire for Americans to be able to be manly and heroic. This desire is most famously connected to Theodore Roosevelt. However, there were other reasons for the imperialism. For example, imperialism was also driven by the desire for overseas markets in which to sell oil and farm products. Expansion was also encouraged by people like Alfred Thayer Mahan who wanted the US to have naval bases around the world from which it could project military might. Zinn then goes on to discuss the mixed reaction to this imperialism from the common people of the US. He notes that many unions supported imperialism because they felt it would be economically good for their members. At the same time, however, there were common people who did not like imperialism because they thought it was being conducted only to help the rich elites. Zinn ends by noting how African American soldiers, in particular, were conflicted about fighting against the Filipinos—non-whites who were only trying to gain independence for themselves.
In all of these chapters, Zinn focuses on the idea that America’s elites have abused those “below” them and have often coopted some of the lower classes to prevent them from uniting against the rich.