In this 2018 video from PBS & You Tube Channel Origin of Everything, written and hosted by: Danielle Bainbridge discusses how the concept of Race came to be in America.
A cited resource is the book ‘Whiteness of a Different Color by Matthew Jacobson.
“Matthew Jacobson writes an overarching history of socially constructed whiteness throughout American history. In doing so he argues that whiteness is not just socially constructed, but that it is also much more fluid, overlapping, and historically contingent than we might think. Broadly, Jacobson charts an initial period in the Early Republic of relative racial inclusion that was based on limiting citizenship to free white people who were "fit for self-government." It was not until the first period of mass European immigration in the 1840s that a second phase began: one that emphasized internal divisions and hierarchies within the white race itself. This system of an internal hierarchy of whiteness was based on a marriage of scientific racism and anxieties over who was fit for republican citizenship. This held sway until the 1920s and the Johnson-Reed Immigration Act, when it gradually gave way to a binary system that expanded the definition of "Caucasian" in opposition to non-whites. This was caused in part by restrictions on European immigrations lessening anxieties and the Great Migration providing an in-your-face oppositional way to define whiteness (even outside the South) against blacks. Ultimately, Jacobson argues that this "Caucasian-ization" of European immigrants was a triumph of progressive liberalism, but points to how it both caused a historical "amnesia" regarding off-white divisions and reinforced a black/white dichotomy.
Within these broadly "glacial shifts" towards a Caucasian identity, Jacobson notes how whiteness was constantly being made, unmade, and often simultaneously affirmed and denied to the same people. In one example, he notes how an Irishman could be completely white for the purposes of naturalization law, an apish caricature in a political cartoon, an off-white Celtic by a nativist, or a frontier protector of whiteness by violently opposing Chinese immigration in the West. In other chapters, Jacobson points to 1877 as a particularly poignant year in which issues of race rose to fore on a number of fronts: from the end of Reconstruction in the South, to growing anti-Chinese movement in the West, to clashes in the Southwest with Indians and Mexicans, to labor disputes highlighting "off-white" European working class, to end of the Great Sioux War on the Plains. He also points to the role of imperialism in laying some of the groundwork for a monolithic "Caucasian" identity. Although much of it occurred during a period of scientific hierarchy of the white race, it also posited a narrative vision that pitted big tent "Caucasians" against non-white "savagery" at America's frontiers, both at home and abroad.”