The Racial Prerequisite Cases
People born in the United States are automatically citizens of the United States. But if you come to the US as an immigrant, and become a citizen, it's called "naturalized" citizenship. In 1790, the US Congress passed a law stating that naturalized citizenship was only open to "free white persons." That is, only a free white person could could apply for, and get, US citizenship. The law left out indentured servants, slaves, and most women, who were considered dependents and thus not free.
It's not always clear the law was closely enforced, because it's full of ambiguity. As you've seen in the Irish immigrant exercise, 19th and 20th century Americans had some very different ideas about what "white" meant.
In the years following the Civil War, a series of "racial prerequisite" cases came before the various state courts and the US Supreme Court. In these cases, residents of the US applied for citizenship, and the Courts were asked to decide whether or not the applicants were "white."
Below is a list of racial prerequisite cases that shows how confused the Courts were--they could not decide the basis for their rulings with any consistency. Review the list and continue.
Are Syrians white? Are Arabs white? Are Armenians white? Are Asian Indians white? What about people who are from mixed marriages.
We'll look a little more closely at two cases in particular, both of which made it to the US Supreme Court: Ozawa and Thind