President Trump wants to end birthright citizenship. Here’s why that will be hard to do.

President Donald Trump signed an executive order Monday night to end birthright citizenship for the children of undocumented immigrants born in this country, a move certain to generate both new controversy and legal challenges.

Trump and other opponents argue that the practice entices people to enter the country illegally, so that children who are born here will automatically gain American citizenship. “That’s a big one,” Trump said as he signed the order.

Those citizens, at age 21, can sponsor close family members to live here permanently. A new poll by Ipsos and the New York Times showed 55% opposition to ending birthright citizenship for children born in this country to undocumented migrants.

It’s the legal foundation under which American citizenship is automatically conferred upon people who are born in the United States. The formal term is jus soli, Latin for right of the soil. Automatic citizenship also extends to children who are born abroad to U.S. citizens.

No. Or at least, not easily. Birthright citizenship is guaranteed in the Constitution, specifically by the 14th Amendment, ratified in 1868, after the end of the Civil War, which says that “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.”

A president cannot amend the Constitution by himself, and a Trump order to end birthright citizenship would doubtless face lawsuits. During his first term, he promised to end birthright citizenship but took little action to do so.

Yes. That principle was confirmed by an 1898 Supreme Court case, United States v. Wong Kim Ark, which held that children born in the U.S. are citizens, regardless of their parents’ immigration status.

“The case set a bright-line rule: When you’re born here, you’re part of us,” Philadelphia immigration attorney William Stock, a former national president of the American Immigration Lawyers Association, previously told The Inquirer.

Wong was a Chinese American cook, born in San Francisco to immigrant parents. In 1890, both parents returned to China, and Wong, about 21, visited them there the same year. On his return, he was admitted into the U.S. on the sole grounds that he was a native-born American citizen.

Four years later in 1894, at about 25, Wong again traveled to China. But when he returned in 1895, he was denied entry by Customs officials who ruled that he was not a citizen and in fact could be immediately deported under the Chinese Exclusion Act. The 1882 Exclusion Act banned immigration from China amid a perceived “invasion” of supposedly job-stealing newcomers.

The Supreme Court affirmed Wong’s citizenship in 1898, citing his birth on American soil. That his parents were not citizens, and in fact were subjects of the Chinese emperor, did not matter, the court ruled.

To deny Wong his citizenship “would be to deny citizenship to thousands of persons of English, Scotch, Irish, German, or other European parentage who have always been considered and treated as citizens of the United States,” the court said.

Very few. Children of foreign diplomats do not automatically acquire citizenship, as their parents’ immunity means they are not “subject to the jurisdiction” of the United States. And were the U.S. to be militarily invaded, children born to members of that “hostile occupation” would not be citizens, either. Native Americans were not automatically granted citizenship until 1924.

It depends. Currently 33 nations and two territories have unrestricted birthright citizenship, and another 32 have some form of restricted citizenship, according to World Population Review.

Nearly all those countries are located in North or South America, stemming from the time when European colonial powers welcomed immigrants in order to populate their settlements. Other countries offer citizenship based on parentage, generally to children where one or both parents are citizens.

Yes. The last 30 years have seen failed state and federal efforts to change the law without having to amend the Constitution, the Center for American Progress reported. In 1995, for instance, a House subcommittee held a hearing on children born to “Illegal Alien Parents” as part of an unsuccessful Citizenship Reform Act, proposed by a California Republican during the Clinton administration.

Testifying for the Justice Department, Constitutional scholar Walter Dellinger explained that while the office grapples with many challenging legal issues, this bill wasn’t one of them. “The legislation,” he said, “is unquestionably unconstitutional.”