President Trump says his executive order banning refugees and immigrants from seven predominantly Muslim nations was not an act of discrimination, based on religion or nationality, but a protective measure against terrorism. That assertion, viewed skeptically by the first judges to consider it, holds the key to whether the decree will survive in federal court.

U.S. law since the 1950s authorizes the president to “suspend the entry of … any class of aliens” whose presence “would be detrimental to the interests of the United States.” But a 1965 law bars immigration preference based on nationality. And the Constitution forbids religious discrimination.

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Stanford law Professor Lucas Guttentag, who specializes in immigration law and is not involved in the lawsuits, said the president’s ban “is transparently and intentionally religious discrimination aimed at Muslims.”