The U.S. Supreme Court in October will hear arguments on the travel ban that the President has called “an important tool for protecting our Nation’s homeland.” But like so many of Donald J. Trump’s assertions, that claim is dubious. It rests upon two premises, neither one consistent with actual experience. Tellingly, many clues indicate that both originated with Attorney General Jeff Sessions, who began pushing them back when he was a U.S. Senator from Alabama, before the president had even been elected. The first premise: Domestic terror is predominantly an imported danger.