Legally, you have to be born here, or be born somewhere else of American parents, or jump through a number of bureaucratic hoops. This isn't about drawing legal lines, though, it's about the bigger question of what makes somebody an American.
Unlike Europe, Asia and Africa, which are made up primarily of nations made up of people who generally look alike, speak the same language, and for the most part had the same religion at some time in the recent past, the United States is a dog's breakfast of creeds, colors, and countries of origin. Our historical roots are in England's Reformation, which produced a bewildering number of Protestant sects that to this day bedevil schoolchildren trying to keep straight the difference between Pilgrims and Puritans. Even in the 17th century, though, there were Dutch and Swedes and French loose in what would become the thirteen rebel colonies (to say nothing of free and slave Afrifcans), with the first Spanish added by the acquisition of Florida.
Still, with all of these admixtures and despite the influx of German and Irish immigrants before the Civil War, the dominant culture was English Protestant. That didn't change until the late 20th century, when Catholics and Jews began to assert themselves politically even as the mainline Protestant denominations had begun to hemorrhage members and influence. The Baptists and other evangelical sects absorbed some of those people but despite some panic among the coastal elites, they never had the kind of influence enjoyed by the Episcopalians, Methodists and Presbyterians.
So we don't have "a" culture to bind us together. We don't have a single religion (even atheists and agnostics are tolerated) to unite us; the archbishops of various Christian faiths are treated with as much (or as little) respect as popular television evangelists and revival preachers such as the late Billy Graham.
All we have, as James Lileks sagely observed yesterday, is an idea. The idea is that we have a law to hold us together, a law based on a Constitution written so long ago that most of our allies at the time have gone through several changes of government structure since then. France has gone through two empires, five republics, and a puppet regime; Japan has gone from military dictatorship under the Shogunate to Prussian monarchy to parliamentary democracy with a figurehead Emperor; the Turks moved from the Ottoman Empire to an aggressively agnostic republic. Australia, Brazil, Canada, Germany, Greece, India, Iraq, Israel, Italy, Korea, Mexico, Mongolia and Singapore (to name but a few) did not even exist. Yet the constitution endures, amended periodically and with great difficulty when we reach the consensus that changes need to be made - or unmade, on occasion.
I think this is a big part of what's at the root of the bipartisan outrage at Congress and the President over the horrible mishmash of an immigration bill that our "representatives" tried to foist on us recently. The 12 million illegals from Latin America (and elsewhere) not only come from a radically different historical-political culture than the rest of us*, they began their stay here with an illegal act. This got up a lot of peoples' noses, especially when it was seized on by the New York Times and normally sensible people like Linda Chavez to make opponents look like a bunch of gap-toothed hillbilly racist hicks. It was a complicated reaction, in and the media did us no service by boiling it down to "bitter, backward nativists v. compassionate cosmopolitan reformers". It involved a lot of things people don't really think about a lot on a daily basis, but deal with at a gut level.
That's enough of that. I need to sort books and do laundry. Later.