The Fourteenth Amendment to the US Constitution declares that anyone born on US soil "and subject to the jurisdiction thereof" is an American citizen. It was intended to give freed slaves guaranteed citizenship in the wake of the 1861-65 Civil War. But today, it also means the children of illegal immigrants to the US automatically become American citizens. This places it right at the heart of the huge controversy over immigration. Adam Smith, historian of 19th Century America, travels to Washington DC and North Carolina to find out.