Howard Zinn’s main argument in this book is that the United States has been a country that has historically abused its working class, along with other groups like women and people of color. Because of this, it is not surprising that he calls Chapter 10 “the other Civil War” because he says that US society was waging a war against the working class.
Zinn follows what he sees as class struggles dating from the 1830s through the 1860s. He argues that the government and the society of the US oppressed the working class during this time. He makes this argument even though this was an era, the Jacksonian Era, that is typically seen as the “era of the common man” in courses on US history. Zinn argues that the sorts of moves that Jacksonians made toward democracy were more of lip service. They really only helped to keep the working class from understanding what its class interest was and rising up against its oppressors. As Zinn says
The Jacksonian idea was to achieve stability and control by winning to the Democratic party "the middling interest, and especially ... the substantial yeomanry of the country" by "prudent, judicious, well-considered reform." That is, reform that would not yield too much.
Zinn calls this chapter the “other Civil War,” then, because he believes that the ruling class in the US spent the antebellum years trying to create a society that they would dominate and in which the workers would be exploited.