In my American legal history unit at the University of Sydney, after a couple of weeks of contextualising the legal, political, social, and intellectual world in which it was written, we spend a full week delving into the original text of the US constitution.

Beyond the nuts and bolts of American governance, most of this time is spent on the indeterminate language of the supreme law of the United States. What exactly does "domestic tranquillity", "general welfare", "commerce … among the several states" or "high crimes and misdemeanours" mean? What about "cruel and unusual," a "well regulated militia" or even "arms?" What constitutes assembling "peaceably" and what would make a search of your person or home "unreasonable"?

The point of this exercise is twofold. Generally, it is to show students that the world's oldest rights-based constitutional system is imbued with deep and intentional ambiguity.

More specifically though, I'm hoping students pick up on a specific lack of clarity in the original 1789 constitution and its first 10 amendments (the "Bill of Rights") added in 1791 — the absence of any definition of citizenship.

The word or its derivatives appears 12 times in the original text and first 10 amendments. Citizens can run for certain offices provided they meet specific qualifications and are entitled to the "privileges of and immunities" of other citizens.

But who are citizens? How does one become one?

Fallacious decision blocked some citizenship

The stakes of this question became especially clear in 1857.

That year, writing for a 7-2 majority in Dred Scott v Sandford, US Supreme Court Chief Justice Roger Taney declared that despite being citizens in several states, African-Americans could not be citizens of the United States — despite no such statutory, let alone constitutional, language.

In what is generally considered the worst-reasoned and most prima facie fallacious Supreme Court decision in American history, Taney's ruling galvanised anti-slavery northern public opinion and the electoral strength of Abraham Lincoln's newly founded Republican Party.

Eight years and more than 600,000 dead Americans later, the insurrection of southern states in direct violation of Article I, Section 10 of the constitution had ended and slavery was abolished.

If you are born in the US, you are a citizen and entitled to all the privileges and immunities that entails. ( Reuters: Mike Segar )

Today's language crystal-clear

The question raised by Taney's decision in Dred Scott of what constituted American citizenship remained unresolved.

On July 28, 1868, the 14th Amendment to the US constitution was ratified. In Section I, the amendment states:

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

Who is a citizen and entitled to citizenship rights was no longer ambiguous whatsoever. If you are born in the US, you are a citizen and entitled to all the privileges and immunities that entails and the equal protection of the laws.

Just in case there was any question, in 1898 the Supreme Court — at the time no friend of racialised minority populations — affirmed the citizenship of Wong Kim Ark, a man born to Chinese parents in San Francisco in 1873. In the 120 years since US v Wong Kim Ark, no judge or serious legal scholar has questioned this reading of the amendment's crystal-clear language.

Where does that leave Trump?

Thus, when Donald Trump last week suggested he could and would end birthright citizenship by executive decree, the sensible reaction is to ignore his statement as the clear xenophobic electioneering stunt that it is.

Mr Trump can do no such thing. Only an amendment to the constitution approved by two-thirds of each of House of Congress and three-quarters of all states can do so — utterly unimaginable in today's political environment.

Given this context, I must admit to a general squeamishness about writing this. Donald Trump wants people to talk about this impossibility for electoral reasons.

But, upon reflection, I kept returning to the second half of my American legal history unit. For the last seven weeks of the semester, we study how those 80 brief words of the 14th Amendment are the lynchpin of civil rights for African-Americans, women, and gays and lesbians, among others. How within them is the roadmap for ending police brutality, sexual harassment, a more robust defence of reproductive rights, and preventing voter suppression.

Unlike Australia, these rights are constitutional principles. Birthright citizenship and its corollary of equal protection is the basic constitutional right that would prevent things like former prime minister John Howard's suspension of the Racial Discrimination Act.

With no such enshrined and unequivocal right of citizenship, Australia is dependent, to a large degree, on the goodwill of the majority to protect the rights of minorities.

In an era of Donald Trump, Jair Bolsonaro, and Viktor Orban, affirming the significance of constitutional birthright citizenship and equal protection seems as important as ever.

Dr Thomas J. Adams is a senior lecturer in history and American studies and academic director of the United States Studies Centre at the University of Sydney.