I have a previous answer on this, which I'll paste below:

"Italians/Irish/Poles weren't considered white" is a popular way of describing the complicated situation in the 19th and early 20th centuries. There's even a book titled How the Irish Became White! However, this framing is presentist: today, most Americans (not so much Europeans) tend to see ethnicity and "race" as equivalent, with the options being "white", "Black/African-American", "Native American/Indian", "Asian", etc. In order to get the modern American to see that Catholic Italian immigrants were not seen as being on the same order as Protestants with English ancestry, then, it's easy to say that Italians "weren't considered white". But this upholds our modern, American ethnic/racial distinctions as objectively real.

The earliest Irish immigrants to America tended to be Protestant, mainly "Scots-Irish" from Ulster; various English-American colonies actively sought Irish Protestant immigrants to come in and start independent farms while snubbing or actively disallowing the Catholics. For instance, South Carolina was officially an Anglican colony, but gave protections to the Scots-Irish who were part of the Church of Scotland. And it wasn't just Irish Protestants - Queen Anne of England promised land to any German Protestants who wished to settle it, prompting a mass movement of poor families from the Palatine region to London and then eastern-central New York. Increased Irish Catholic immigration came during the potato famine of the 1840s, and there was a fresh wave of German immigrants as well - not spurred so much by the revolutions of the late '40s, but because they were also affected by the potato blight and wanted to escape poverty. (About 1/3 of these German immigrants were Catholic themselves.) Italian and Polish immigration mainly started in the last quarter of the century, and likewise involved mainly Catholic rural laborers from poor areas, coming with very little besides themselves. Most ended up living in tenements, working in factories or sweatshops or as laborers in construction projects, which in and of itself would lead middle- and upper-class America to look down on them.

Americans with English heritage, often descended from pre-Revolutionary colonists or from immigrants from early in the 19th century, saw themselves at worst as the default, and at their most self-aggrandizing, a superior form of humanity. Protestant > Catholic, and certain flavors of Protestantism were better than others. Northern Europe > Southern Europe; Western Europe > Central or Eastern Europe; England > Scotland > Ireland. Stereotypes of these white immigrants abounded, usually depicted with painful eye-dialect in writing or in thick accents on the stage; they wouldn't always be written as negative characters, per se, but the humor came at the expense of how Other they were from "normal" Americans. And things got uglier with the Know-Nothing movement, a nativist group/party that organized against immigrants because "they're lazy", "they're taking jobs", "they're outnumbering good Anglo-Saxon stock", and the other xenophobic fears that certainly were not confined to that one moment in time. Newspaper ads did indeed discriminate, as discussed by /u/sunagainstgold here, on the basis of origin and religion, because the ideal and most prestigious servant was white, English, and Protestant. (Many had to compromise. The stereotype of the Irish cook or housemaid was quite prevalent.)

However, while some did make statements equating the Irish and African-Americans in the early 19th century (largely before large-scale immigration from other countries), there was a very solid difference between black Americans and any European immigrant groups: slavery. As /u/freedmenspatrol discusses very adeptly in this answer, the entire concept of being a free white man required the opposition of the unfree state, resting on the real or theoretical ability to enslave black men, women, and children. Pre-Civil War, no matter how poor any immigrant from Europe was, they were still "free". Even after the Civil War, the ideology persisted.

Because what "become white" means in this case is that the way white ethnicity was perceived changed. Immigrant groups - first the Irish and Germans, then the others - made inroads into local government and started their own businesses, changing the narratives around their stereotypes and taking power. They became acculturated, holding onto their ethnic identities while learning to navigate America, and gradually an individual's parents' country of origin within Europe meant less to outsiders. Whatever place in Europe your family had come from, in the United States you could be just another white person after World War II, though some stereotyping would persist. It's not a coincidence that this happened as organized protest about the status of African-Americans began to rise, setting up a more important dichotomy than English-American vs. Irish-American vs. Italian-American.

Some sources you might be interested in, though I referred to others as well in writing this:

The Irish Way: Becoming American in the Multiethnic City, James R. Barrett (2012)

Nativism and Slavery: The Northern Know Nothings and the Politics of the 1850's, Tyler Anbinder (1992)

Polish Refugees and the Polish American Immigration and Relief Committee, Janusz Cisek (2006)

"people with a critical race perspective"

I don't mean to call out OP, but I mostly want to use this as a point to just note that Critical Race Theory is a very specific concept of legal theory that has been around since the 1970s, and is considered to have mostly started with Derrick Bell. u/EdHistory101 has a great answer with some background on the subject here.

I just want to point this out because the term "critical race theory" or "critical race [whatever]" has become something of a bugbear because of contemporary politics, and seems to be seeping into common vocabulary, but we should be clear that Critical Race Theory is a very specific thing that is not synonymous with "discussing the role of racism in US history".

I posted a related answer on another account some years ago, which you can find here.

This is a complex question, and there isn't really one way that non-Anglo Europeans became "white" in the United States. I am by no means an expert, but I can recommend two books that I've found quite helpful in thinking through this:

• Nell Irvin Painter, The History of White People (Norton: 2010)
• Matthew Fyre Jacobson,Whiteness of a Different Color: European Immigrants and the Alchemy of Race (Harvard UP: 1999)

I think there are a couple of important caveats to any discussion like this. First, there is a historical distinction between "whiteness," the Caucasian "race," and Anglo-American conceptions of what it meant to be of the American race. Skin color was not always the sole or even primary indicator of racial difference, although it has certainly come to be. To give you an example, in 1899 William Ripley published a book called The Races of Europe: A Sociological Study in which he collated huge troves of data from various European anthropologists into an overarching theory of biologically determined racial difference for the continent. He divided Europe into three racial types: the Teutonic, the Alpine, and the Mediterranean. And these three races were distributed across Europe more or less north to south (Teutonic in the north, Alpine in the middle, and Mediterranean in the south). The basis of this division was not due to culture, or skin color, or any of the markers we know today. The division was based on primarily on head shape ("the cephalic index"). Here's a map of the index from Races of Europe. Madison Grant, in his book The Passing of the Great Race, took Ripleys theory and made it fully normative, equating the Nordic (Teutonic) race with human excellence.

Nineteenth century racial theory typically grouped these European races into one larger racial category (Caucasoid), which was set apart from the Negroids (African) and Mongoloids (Asian). Polygenism was the theory, popular throughout the 19th century, that these three races were in fact not biologically related to one another. And noted scientists like Louis Aggasiz spread polygenic racial theory across the country on the lecture circuit. I mention this because it's important to realize that racial differentiation within the Caucasoid race was of an entirely different magnitude than the racial differentiation between Caucasoids, Negroids, and Mongoloids. And although thinkers like Grant would magnify the racial differences within the larger Caucasian classification, that was peculiar to his style of eugenic theory, not all racial theory. The racial divisions within the caucasion race still exist today, but rather than "racial difference" we now talk about them as "ethnic difference." And although ethnicity is now largely understood to index culture, many people still describe ethnic difference in terms of biological difference (e.g., swarthy Italians).

I mention all of this to suggest that, although many scholars talk about X or Y group "becoming" white, whiteness was not necessarily a category that existed from the start and slowly accumulated a larger population. We can also think about whiteness subsuming other kinds of racial/ethnic difference--in other words, it's not that Germans in America became "white," rather, their whiteness became their most defining feature. Jacobson argues that "the contending forces that have fashioned and refashioned whiteness in the United States across time . . . are capitalism (with its insatiable desire for cheap labor) and republicanism (with its imperative of responsible citizenship)" (p. 13). To this list I would also add religious toleration, since anti-semitic and anti-catholic sentiments have often governed who was white. But that does not explain why whiteness replaced European racial divisions as a primary mechanism of making sense of social difference.

What most scholars mean when they talk about whiteness in American culture is the suturing of skin color to "Anglo-American" culture, which is also difficult to quantify. Elisabeth Kinsley, for example, talks about "ethnic" performances of Shakespeare in New York City between 1890 and 1910 as one path through which ethnic difference was reconciled with Anglo-American culture (Kinsley, "This Island's Mine: Mapping the Borders of Shakespeare, Whiteness, and National Belonging in Manhattan's Ethnic Theaters, 1890-1910" Text and Performance Quarterly 34 (2014): 52-71), and there are literally countless other ways in which this kind of cultural refashioning took shape.

But "becoming Anglo" doesn't tell the complete story either. Jacobson argues that three forces were particularly important in allowing whiteness to coalesce as a unified racial category: American imperialism, which "conferred its benefits by a logic of pan-white supremacy,"; naturalization case law related to changes in immigration law in the 1920s; and civil rights politics "[eclipsing] the lingering divisions among the white races as it pressed its agenda of racial justice defined by the binary logic of the Jim Crow south." (201). He concedes that this list is far from exhaustive, which speaks to just how complex this question would be to answer, but he sees these three forces as crucial.

I would recommend reading his book to learn the specifics, because the arguments are complex and the textual archive he marshals is very deep. Painter's book tells a very similar story, although she has a more nuanced look at permutations within whiteness in the early part of the 19th century. Where Jacobson focuses heavily on policy, law, and science, Painter talks about the cultural currents that made whiteness what it is.

I'm a little confused by some of the answers here. I have studied Italian history and there was great segregation in Italy before the diaspora of Italians. Some Italians who came (from southern Italy) pretended to be from Mexico in order to work places due to their skin being brown. Italy over decades of course has had more northern Europeans travel to Southern Italy and became more predominantly fair in skin tone.

Ofc, this does not excuse that racism is an issue at an institutionalized level. I've also heard that Italians and Irish can be the most racist. And I would like to know more about these issues.

Would love resource titles.