Tenzer begins with the legal definition of slavery and of terms such as White, Black, and mulatto (which often differed from the social definitions). The partus sequitur ventrem rule made the offspring of a slave mother slaves, regardless of their colour. (No slave could be White, of course, so white slaves were classified as mulattos.) Chapter two looks at the consequence of this rule, the presence of white slaves in the South. Tenzer makes no attempt to provide quantitative figures here, stressing instead the accessibility of accounts of white slaves in the North (notably advertisements for runaway slaves who could "pass" as white). However many of them there actually were, the idea of slaves indistinguishable from free whites was widespread in the North.
Chapter three looks at Southern racial theory, in particular the fabrication of figures for insanity in the 1840 census and Dr Nott's idea that mulattos were unhealthier and shorter lived than black slaves. This leads to a chapter on the illicit slave trade, which Tenzer argues is the explanation for census results showing an apparently higher "fecundity" for black slaves than for free blacks and mulattos. His argument for an extensive illicit slave trade (continued in an appendix) is indirect but persuasive.
The 1850 Fugitive Slave law allowed runaway slaves to be reclaimed without due process, creating the possibility that free whites could be seized accidentally, or even kidnapped. This was perceived as an attack on freedoms inside the North and many states passed personal liberty laws in response. The political power of the South and events such as the destruction of the Missouri Compromise and the Dred Scott decision also raised fears of slavery being extended into the territories and Northern states. Mixed with ideas that "capital should own labor" and that slavery was right, regardless of colour, this produced an explosive atmosphere. However seriously leaders in the South may have contemplated the nationalisation of slavery or the possibility of enslaving free white laborers in the North, there was enough evidence for this to make it a major theme in anti-slavery campaigns and Republican political propaganda.
Detailed references and some of the argument are left to the endnotes, and The Forgotten Cause of the Civil War is accessible to the non-specialist — despite having only a slender background knowledge of the period I had no trouble following it. I found Tenzer's thesis convincing: it resolved my perplexity about a war being fought to end slavery without blacks being granted civil rights. In any event, The Forgotten Cause of the Civil War, with its extensive quotations from newspapers and other texts of the period, both Southern and Northern, paints a vivid picture of attitudes to slavery in the decades before the Civil War.