Most Americans believe birthright citizenship is the global norm. How many times have I heard people joke, "you should have a kid while you're living in Europe because then they'll have EU citizenship!" Or, "would you have a baby when you're in Australia? At least you'd get free medical care and they could probably go to University there for free when they're older."
In other words, and not very surprisingly, most people in the US have no idea that we in the Americas are almost unique in our ongoing support for birthright citizenship.
In the United States, birthright citizenship is very clearly established by the 14th Amendment and upheld by the Supreme Court. The 14th Amendment says that "all persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and the State wherein they reside."
But there is renewed debate within the US as to whether we should continue to recognize birthright citizenship for "illegal" immigrants. To some extent, this debate is difficult to parse, as most immigration to the United States has been "illegal" in the sense that people just showed up on boats without visas. These immigrants were variously welcomed or disenfranchised, depending on the state of the nation at the time and the racial views of the ruling elites. Chinese immigrants, for example, were excluded from citizenship until the 1940s.
In 1857 the Dred Scott ruling formally stripped all black people in the US of their citizenship. The decision was highly controversial as it stripped free blacks of their citizenship in states where slavery was illegal, essentially creating a massive population of stateless Americans across the country.
One of the primary jobs of the 14th Amendment was to permanently restore citizenship not only to blacks, but to all persons born in the United States...and subject to its jurisdiction. In recent times, a new controversy has emerged as to the meaning of the later phrase. It is widely accepted that the later phrase excludes, for example, the children of foreign diplomats.
In Wong Kim Ark, the US Supreme Court interpreted the 14th Amendment as granting citizenship to all persons born on US soil to immigrant parents, applying the principle of jus soli, or law of the soil.
But Wong Kim Ark's parents were legal immigrants to the United States. The Court has never directly ruled on the issue of "illegal" immigrants. Were the Supreme Court to rule that children born to undocumented immigrants in the US are not citizens, we will once again see the creation of a massive group of stateless persons on American soil.