The U.S. Supreme Court late Wednesday gave the Trump administration permission to enforce its toughest restriction yet on asylum seekers at the southern border, even though a lawsuit to stop the new policy is still working its way through the lower courts.

As a result, the government can now refuse to consider a request for asylum from anyone who failed to apply for it in another country after leaving home but before coming here. As a practical matter, it means that migrants from Honduras, Guatemala, and El Salvador cannot seek asylum in the U.S. if they didn't first ask for it in Mexico.