While presidents in the modern era have declared dozens of emergencies to address various problems, none has been remotely as disputed as the one Mr. Trump is contemplating. Mr. Banks said he had found no examples of a lawsuit in which someone tried to challenge the factual basis for a president’s determination that an emergency existed, leaving scant guideposts for what courts might do with any legal challenge…

Critics note that the number of people crossing the border illegally is far lower than it was a generation ago. The relatively new phenomenon of caravans of migrants consists largely of families who present themselves to border officials and request asylum, rather than trying to go deeper into the interior on their own. Most illegal drugs are smuggled in through ports of entry. And there has been no instance in the modern era of a terrorist attack on domestic soil that was committed by someone who sneaked in across the southern border with Mexico.

Still, the Justice Department would surely argue that courts should not even consider the facts, but instead should defer to the president’s determination that an emergency exists. There is a long history of courts being reluctant to substitute their own thinking for the president’s in security matters — or declaring that it is a “political question” for the two politically elected branches to work out between themselves.