President Trump just opened another front in the war that defined his candidacy from the start. Not the decision to send troops to the southern border to deter a caravan of Central American migrants from entering the United States, but his call to eliminate birthright citizenship.

It's a radical move that would profoundly reshape the very definition of "American." And rather than trying to ward off that change by waving around the 14th Amendment — which enshrines birthright citizenship in the Constitution — like a totem, it's worth explaining why.

Defining citizenship by place of birth — jus soli — rather than only by blood descent — jus sanguines — feels distinctly American. But that birthright, contrary to President Trump's assertion, isn't America's alone. Unconditional or nearly unconditional birthright citizenship is a feature of most countries of the so-called New World, including Canada, Mexico, Peru, Brazil, Argentina, and all the countries of Central America.

The obvious reason: The overwhelming majority of the population in these countries traces its descent to settlers and immigrants, people who came from elsewhere, and who already had some other nationality. If "Americans" are a people at all, it's because we are in this place together, and have made a collective identity here together. The peoples who have a rationale to restrict membership to those with the proper lineage are groups like the Cherokee Nation or native Hawaiians, who were here first, and who want to preserve a communal identity in the absence of territorial control. "Americans" are not such a group.

But jus soli has roots even deeper in our heritage, roots that go back to our mother country. Britain began to define itself as a nation in a modern sense in the 17th century, and its conception of membership wasn't one of citizenship but of subjecthood — subject to a crown that unified the English and the Scots and defined by the territory over which that crown held sway. While your ethnicity (and your title) was a matter of blood, your membership in the polity was a matter of where you were born.

That's the conception of membership that came to America with the British colonists — at least for white people. Whether you came from Britain, France, the Netherlands, or Germany, whatever your language or your religion, if you were born in New York you were a New Yorker. And when America achieved nationhood, that citizenry moved from self-government — which we largely had as colonies — to full sovereignty and independence.

Which is a very different experience from most of the nations of the world which follow a jus sanguinis approach to defining citizenship. Most of these nations came into being during the 19th century age of nationalism or the 20th century era of decolonization. Their conceptions of themselves as nations were forged by opposition to a foreign intrusion: the German peoples' reaction to conquest by Napoleon, Indians' reaction to conquest by Britain, etc. They had to begin by defining who they were because it was on that basis that they could come together to be a nation in the first place.

In Europe in particular, many of these new nations, when they were established, had territories that only loosely corresponded to the boundaries of the peoples they claimed to represent. Countries like Hungary, Germany, and Russia — and Israel — sought to knit large diaspora communities together with the homeland. Jus sanguinis was a way of binding the nation (as they defined it) together.

None of that applies to America. On the contrary: The purpose of eliminating birthright citizenship would not be to bind the country together, but to tear it further apart. The undocumented population that is excluded from the protections of citizenship would swell, as their children and grandchildren failed to achieve membership either, forming a helot class ripe for multi-generational exploitation. Germany has already had experience with precisely such a development, as its restrictive citizenship laws kept the children of Turkish guest workers outside the body politic. It was to remedy this situation, and pull the Germany that actually exists closer together, that the country eventually liberalized its own citizenship laws, bringing them more into line with France and the U.K.

Whether or not Trump's executive order is permitted by the Constitution, allowing America to become further divided by citizenship would be a clear violation of the spirit of the 14th Amendment. That amendment was passed, after all, precisely to overturn the Dred Scott decision, and to make it clear that, with the abolition of slavery, the freed slaves would not remain outside the boundaries of citizenship, but would be integrated into the body politic.

But it's abundantly clear that a blanket revocation of birthright citizenship would be completely unreasonable interpretation of the 14th Amendment, at least according to any originalist or textualist or other conservative reading of the document and its legislative history. Undocumented immigrants may be breaking the law, but they are clearly and plainly subject to the law — which is what "subject to the jurisdiction thereof" in the amendment means. Members of sovereign Indian tribes or the children of foreign diplomats were excluded from citizenship because they are subject to an extraterritorial jurisdiction of another power. That is not the case with undocumented immigrants.

Trump's fulmination against birthright citizenship makes much of phrases like "birth tourism" and "anchor babies" — and these are real phenomena. Trump should know, since his Miami condos are favored destinations for wealthy Russians looking to confer the benefits of American citizenship on their offspring, and a similar industry in Los Angeles caters to wealthy Chinese families. Birth tourism does cheapen the privilege of American citizenship, and a number of countries — like the U.K. and Australia — have modified their citizenship laws precisely to reserve citizenship for people who intend to actually live in the country.

But the vast majority of undocumented immigrants do precisely that. And if the aim really is to curtail an industry aimed at wealthy collectors of multiple passports, there are surely regulatory fixes — crippling fines for soliciting non-citizens to give birth on American soil, say — that could be applied short of redefining citizenship itself.

That definition, legally embodied in the 14th Amendment but conceptually stretching much farther back, is part of our own birthright. We shouldn't sell it for political advantage, or to enhance our perceived status, or for any other mess of potage.