I am lead counsel in the challenge to Donald Trump’s birthright citizenship executive order. As I and my team help the ACLU legal director, Cecillia Wang, prepare for the supreme court argument in this case on Wednesday, we are poring over legal minutiae and sharpening our arguments. But the larger questions that loom over the whole case are simple: What does it mean to be an American? Will we adhere to the best of American history and protect the values of equal citizenship and opportunity?

In early America, like today, people born on US soil were citizens, even if their parents were immigrants. That’s a principle we inherited from England as part of a body of rules known as the “common law”. In England, that rule was originally about monarchical power; but in our young republic it found new life as a principle of equal citizenship. As waves of immigrants arrived, the birthright rule ensured that the child of Irish or German immigrants would be no less citizens than those who traced their lineage back to the Mayflower.

Before the American civil war, this principle of equal citizenship was woefully incomplete. The founders declared that “all men are created equal”, yet the original constitution tacitly accepted the sin of slavery and counted enslaved human beings as “three-fifths” of a person.

These two contradictory ideas – equal citizenship of everyone born in this country, and the exclusion and subordination of a large part of our nation – came to a head in the supreme court’s shameful decision in Dred Scott. The court rejected the traditional birthright rule when it came to Black Americans, claiming that they could never be US citizens even if they were born free on US soil. The decision helped precipitate the civil war.

After the war, Congress drafted the constitutional provision at issue in Trump v Barbara – the citizenship clause of the fourteenth amendment – to eliminate Dred Scott’s mistake and instead enshrine birthright citizenship into the text of the constitution. That clause guarantees that “all persons born” in the US are citizens, except for people such as ambassadors’ children who are immune from “the jurisdiction” of the United States. Congress secured the birthright principle in the constitution to head off future denial of birthright citizenship by any branch of government.

It’s no mistake that Congress, through the citizenship clause’s words, enshrined universal birthright citizenship. The same Republican Congress that sought to guarantee “equal protection of the laws” to everyone in the country embraced the longstanding birthright citizenship rule as an egalitarian principle matching their vision of a nation of free and equal citizens. The framers of the citizenship clause recognized and specifically endorsed that universal understanding.

Not everyone saw things that way. For example, some in Congress objected to the fact that the children of Chinese immigrants would be made citizens.

In 1898, that issue came to the supreme court. Amid extraordinary bipartisan anti-Chinese political sentiment, the government recycled Dred Scott’s concept of exclusionary citizenship, albeit with a new target: the children of Chinese immigrants. The supreme court flatly rejected this effort to distort constitutional text and history. In no uncertain terms, it confirmed that the citizenship clause adopted the egalitarian, universal rule: if you are born in this country, you are a citizen, regardless of your parents’ citizenship or immigration status. That rule has been understood and applied for 128 years, providing stability and security for generations of families and serving as a cornerstone of our modern American nation.

Today, that principle is under attack once again. Trump’s executive order targets babies whose parents lack permanent immigration status. It strikes at children of diverse families around the country, with parents here for years or even decades on work and student visas; those on Daca status who have lived in this country since their own childhood; and those seeking protection here from persecution, unrest and natural disasters. The order seeks to deny citizenship from tens of thousands of babies every month, and casts a shadow over the citizenship of millions more of our fellow Americans.

The details are different, but the executive order’s vision of America traces directly back to Dred Scott and the anti-Chinese arguments raised and rejected in US v Wong Kim Ark. Trump wants to create a nation divided by parentage – one where children can be born in this country, live their whole lives here, and yet be completely excluded from all the rights and duties of full membership in our society.

That is not the America we know and love. In our America, every child born here is an equal citizen. And that is ultimately what the Barbara case is all about. Will we reinforce our shared values of equal citizenship, inclusion and opportunity? Or will we turn the clock back to the exclusion and division of Dred Scott?