The Wednesday case stems from an executive order Trump signed on the first day of his second term ending what’s known as birthright citizenship, which guarantees citizenship to nearly everyone born on U.S. soil.
While the concept has been part of U.S. law for well over a century, it is relatively rare around the world.
Most countries follow the principle of jus sanguinis, or “right of blood,” with a child’s citizenship based on the citizenship of their parents, no matter where they are born.
None of the 27 member states of the European Union, for example, grant automatic, unconditional citizenship to children born on their territories to foreign citizens. The situation is similar across much of Asia, the Middle East and Africa.
Some countries use a combination of principles, such as parenthood, residency and ethnicity, to decide a child's citizenship.
Australia, for example, allowed birthright citizenship until 1986. But starting that August, children born there could only become citizens if at least one parent was an Australian citizen or a permanent resident.
What is the Trump administration’s argument?
Supporters of birthright restrictions in the U.S. focus on a handful of words in the constitutional amendment: “subject to the jurisdiction thereof.”
That phrase, they argue, means the U.S. can deny citizenship to children born to women who are in the country illegally.
A series of judges have ruled against the administration and the order has been repeatedly put on hold by lower courts.
Wednesday's case originated in New Hampshire, where a U.S. district judge ruled the order “likely violates” both the Constitution and federal law.
