In holding that Mr. Trump acted in bad faith, the Fourth Circuit fundamentally misconstrued Justice Kennedy’s opinion in Din, which nowhere suggested that, once the government had articulated a facially legitimate purpose, the courts could weigh whether there might have been an additional, improper purpose. As the Fourth Circuit dissenters explained, Mandel requires only a facially legitimate and facially bona fide reason.

Any other standard would constitute an invitation to the judiciary to direct the nation’s foreign and defense policies. Having misapplied Din, the Fourth Circuit went on to apply a standard domestic case-law analysis, under which the existence of a discriminatory purpose essentially dooms the exercise of governmental authority irrespective of other justifications. Under that approach, the government would have lost in Mandel, Fiallo and Din.

If the Fourth Circuit’s reasoning were to stand, it could cripple the president’s ability to defend the country. The judges claim Mr. Trump’s campaign statements, supposedly hostile to Islam rather than Islamist terror, transform his order into an “establishment” of religion in violation of the First Amendment. If the president is forbidden to impose temporary limitations on immigration from any Muslim-majority nations, it would follow that he is prohibited from taking any hostile or unfavorable actions, including the use of economic sanctions or military force, toward any Muslim-majority nation.