Race is an idea that has not always been an explanation of human difference. Early forms of difference that influenced the idea of race were part of the Spanish Inquisition and the English treatment of the Irish. Race solidified as an idea with colonization and a unique type of slavery in the sixteenth and seventeenth centuries and through ideas about Manifest Destiny, scientific racism, and taxonomic notions about categories of people.
Native people experienced population decline through the ravages of epidemic disease, physical violence, and slavery. Different European groups followed their own strategies of colonization. Spanish colonization was led by Conquistadores who, although they admired the advances of the city of Tenochtitlán, still demolished it. The English used religious thought as a means of determining their superiority and interpreted the epidemic disease faced by Native groups as ordained by God. A constant oppressive experience for various Native nations consisted of the loss of their land through removal, treaties, and governmental actions. African-descended peoples first came to the United States to work on sugar cane plantations. Slave traders forced 10 to 30 million Africans in captivity to come to the United States. On the East Coast of the United States, white society treated slaves kidnapped by the English, Dutch, and Spanish for the slave trade as similar to indentured servants. Shortly after 1660, the institution of slavery of black people by whites developed into a harsher system through the eventual passing of Slave Codes where people were classified, treated as property, and born as property, with no end to slavery during their lifetimes. Their status was as less than human. Even if whites did not own slaves at the time, they took part in the economy that included and profited from slavery. In fact, profits of the slave trade partly subsidized the American Revolution. A larger goal of white elites consisted of encouraging working-class whites to reject alliances with African and African Americans to prevent uprisings based on class interests. The creation of the idea of race and racial superiority aided in those goals, as well as being an excuse for the continuation of African American slavery. The first theories of race emerged through pseudoscientific classifications and testing, such as brain size and skull measurements.