Reginald Horsman begins Race and Manifest Destiny stating, “By 1850 American expansion was viewed in the United States less as a victory for the principles of free democratic republicanism than as evidence of the innate superiority of the American Anglo-Saxon branch of the Caucasian race." He lays out a compelling case that American thinking about race shifted dramatically in the period between the War of 1812 and the Mexican-American war. Going from a country in which the voting franchise was restricted to a propertied elite to one in which the only requirements for voting were gender, age, and race. This idea was derived from a desire to justify the consequences of American greed upon African and Native Americans and reinforced by, “the survival and prosperity of the tiny colonies elaborated by the miracle of a successful revolution against the might of Great Britain and confirmed by a growth that amazed the world in 60 years after the conflict.” This formulation creates the idea of America as a White, Anglo-Saxon, Protestant nation “destined to bring good government commercial prosperity and Christianity to the American continents and to the world.” This “Manifest Destiny” has been tied to ideas about the American West ever since.

To paraphrase Horsman America went from one in which a propertied voting elite believed that prosperity and freedom of liberal democracy could be adopted by anyone given sufficient education to an America where expansion was a byproduct of divine will and neither Mexicans, Chinese, Africans, nor Natives were capable of creating or participating in democratic government which was the purview of only White, Anglo-Saxon, Protestants. This thinking evolved parallel to American expansion in the wake of the War of 1812 and came to define who was and was not a participant in the American nation because, “at the heart of the American and Western European consignment of other races to an inferior lesser human says was the need to justify exploitation and destruction." These ideas according to Horsman don't just pop up in concert with America's westward expansion & it's simultaneous expansion of slavery but were built upon foundations laid in Great Britain.

Horsman splits his book into three sections. The first looking at the European & British origins of those racial theories which developed in the 16th & 17th centuries. He then continues on into ideas surrounding Manfiest Destiny and how those early racialist ideas became foundational to Manifest Destiny. Then concludes with how racialist ideas became embedded into American political ideology of the mid 19th century.

TLDR: Reginald Horsman's Race and Manfiest Destiny investigates the emergence of "scientific" racialism as a justification for first European and later American conquests.