For a long time, leaders (and many citizens) saw the United States as a country of, and for, white people. This is clear in our treatment of Native Americans and our trampling of many of the contracts we drew with them, and obviously, in our treatment of African slaves who then developed an African-American identity long before they were seen as legal citizens. With the powerful, nearly mythological construct of their "Manifest Destiny" backing expansion, Americans blazed across the continent, reaching the West coast and declaring nearly all that land, save for a small portion for Natives, as theirs.
But when it came time to seek other pieces of the Americas, there was a marked reluctance and not nearly the same sense of a God-given right to any land. We like to think that this was our sensible side taking charge, smartly avoiding the mishaps of other more established imperial powers, like England and Spain, whose empires were waning, and we saw the utter hassle of maintaining that much territory. There is a bit of truth in that.
More compelling, however, is this uninterrupted notion of America as a republic for white citizens. The colonization of African Americans back to Africa lay on the table long enough in American policy history to stand as a solid example of just how incompatible some saw a United States that contained large minority groups; need we even mention the Chinese Exclusion Act, which barred Chinese immigrants for twenty years upon its signing in 1882? (Asian immigrants weren't even allowed to earn citizenship until the 1940s.) We've had a treacherous, scarred, love-hate relationship with our immigrants, even while we became known as "the Melting Pot," but this relationship was especially acute when it came to annexing territories not contiguous to the North American continent.
Historian Eric Love argues that racism in the era of American imperialism was not used to support imperial annexation, harking on the "white man's burden," but was actually what anti-imperialists used against that cause. In the cases of Santo Domingo (today the Dominican Republic) and the Philippines, large populations of non-white natives was a large deterrent in making these islands part of the United States. Alaska had no such trouble, as Americans could more easily see it as assimilating with the nation as it was. In the case of Hawaii, that far-flung acquisition, the islands were annexed after enough convincing was done in showing that a large number of whites already lived there, the natives were docile, and populations like the Portuguese were, for all intents and purposes, white already. Plus, they argued, we need to protect those whites already living there.
So whether the Unites States has sought the land that happens to be occupied by non-whites, or whether non-whites have showed up at our doorstep and requested residency here, determining the fine line between citizenship and race has never been easy.
I would like to take this further and consider this fickle, slippery past in terms of the notion of nationality and self-identity. As the twentieth century developed around what started a bit earlier, with the world's structure as all land drawn up and broken into nation-states, we added another layer of personal identification: nationality. Before that the world operated on a more local level, and people's lives were far more contingent on what township you were from than anything else. Also significant was the language you spoke.
Fast forward a hundred years past Americans' fixation with non-whites standing as majorities in any hypothetical new states, and we are still discussing the issue of assimilation and "American" identity, though with far less racist undertones and with more modern logic. (Check into Love's book for some juicy stuff on our imperial era and his argument, which breaks the conventional historical narrative. He backs up his gutsy point with solid research.)
Case in point: the controversy surrounding the hypothetical admittance of Puerto Rico as a state, which, with its current school system and organ