Whenever the French have had a revolution since the first great eruption of 1789, they’ve re-written their Constitution and started over again — so the current French state is the Fifth Republic. Over the same period, in contrast, the USA appears to have had one stable Constitutional order. But appearances can be deceptive. Beneath the veneer of continuity, the US had a major constitutional rupture in the middle of the nineteenth century. The three post-Civil War amendments (the 13th, 14th and 15th) amounted to such a profound re-configuration of the political order that they deserve to be thought of as the practical equivalent of a new, second founding. This matters because this new post-Civil War constitution has shaped American history ever since. Or at least, that’s what I argue in a BBC Radio documentary called The Battle for the US Constitution.

In it, I focus on the 14th Amendment, passed by Congress in 1866 and ratified by the states two years later. The 13th Amendment abolished slavery. The 15th tried to ensure that race could not be used to deny any man (and it was just men) the vote. The 14th Amendment, sitting between the two, was the keystone of the edifice. It defined a national community for the first time and did so in a deliberately inclusive way by saying that if you’re born in America, you’re an American.

All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.

No ifs, no buts in there. No opportunity –or so it would seem — for Congress or state governments to gerrymander citizenship. The ambition of the framers is astonishing: they sought to ensure that men and women who a year earlier had been bought and sold as property were citizens of the republic.

And then, the Amendment spells out what citizenship means:

No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.

The first American Constitution said nothing about what states could or could not do. It was concerned only to restrain the Federal government. But the 14th Amendment says that state governments cannot abridge the basic rights of this new universe of citizens. Indeed it goes further: it says that that a state cannot deny to any person the equal protection of the laws.

Much of US history since 1868 is the story of what this means in practice. Without the 14th Amendment Congress would not have had the constitutional authority to pass the Civil Rights Act of 1964, or the Supreme Court to rule that state laws preventing gay people from marrying were unconstitutional.  Arguments over everything from voting rights to transsexual people’s choice of public bathroom come back to this revolutionary change to the American political order.

…our right to the equal protection of the laws as set out in the first section of the 14th Amendment.

The 14th Amendment also means the children of illegal immigrants to the US automatically become American citizens, which places it right at the heart of the huge controversy over immigration. That’s why Donald Trump wants to abolish the Amendment altogether, and why a Republican Congressman, Steve King, who I interviewed for the programme, wants instead to radically reform the interpretation of the Amendment – so that it no longer gives the children of undocumented migrants the right to a US passport.

But strikingly, the meaning of the Fourteenth Amendment is also fought over in a very different way. This is the  concept of “corporate personhood”, which has been used by the Courts to give corporations the same rights under the 14th Amendment that people have. As  Mitt Romney once said “corporations are people too, my friend”. Hillary Clinton now supports a new Amendment to upend that idea.

For more on the history of the 14th Amendment and why it still matters today, see my piece on BBC History Extra.