A ruling by a federal appeals court against President Donald Trump’s ban on travel from seven countries sets the stage for a potential new conflict between the branches of government.

The unanimous ruling by the Ninth U.S. Circuit Court of Appeals returned a discrimination lawsuit by two states challenging Mr. Trump’s immigration policies to the trial court of U.S. District Judge James Robart in Seattle, where the two sides will gather and exchange evidence in support of their claims.

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The Trump administration can rebut claims of religious discrimination against Muslims with “evidence of a need to halt travel from the seven countries to protect national security,” said David Freeman Engstrom, a law professor at Stanford University.

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The White House could assert a form of the privilege that protects from disclosure communications related to the process by which policies are formulated, said Stanford’s Mr. Engstrom, who studies litigation and regulatory regimes.

The privilege isn’t absolute. Judges must weigh it against the interests for disclosure, such as the evidence’s relevance and the seriousness of the issues involved in the litigation, Mr. Engstrom said.