Thursday’s decision by a three-judge panel of the Ninth Circuit Court of Appeals, which upheld a trial judge’s decision to temporarily stay President Trump’s travel ban, broke no new legal ground. And yet it may well be remembered as one of the most important rulings in modern American history.

Its significance lies in its unanimous reaffirmation of the basic constitutional principle of checks and balances, at a time when that principle is under unprecedented attack.

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The ban itself raises disturbing constitutional questions, as several judges have now found. It bars people from seven majority-Muslim countries from entering the U.S. for a period of 90 days.

Although its stated purpose is to protect the country from terrorist attack, official statistics reveal a dearth of domestic terrorism perpetrated by nationals of these countries. This fact, combined with Trump’s many references to a “Muslim ban,” strongly suggest that the order’s intent is to discriminate against Muslims.

Under well-settled law, favoring one religion over another is unconstitutional, even when cloaked in seemingly neutral terms.

More alarming than the ban, however, was the administration’s claim that the courts could not review its legality.

Justice Department lawyers pointed out that the Constitution gives Congress and the executive branch authority over immigration. They also noted that the president is entitled to great deference in his national security judgments. In both assertions, they were on solid ground. But they took their case one giant step further, arguing that the president’s action was “unreviewable.”

JUST IN: Appeals court unanimously rules against Trump, won't reinstate immigration ban https://t.co/uzsVCLNODl pic.twitter.com/Mj95W7oZ7G — The Hill (@thehill) February 9, 2017

In one sense, they had to make that argument. The order could not withstand even the gentlest scrutiny. The administration offered no evidence that would justify the ban as a security matter. Indeed, the only probative evidence on this point was a declaration by prominent former national security officials, submitted by the State of Washington (which had challenged the ban), explaining how the order would make the country less safe.

The lawyers’ solution, however, was ill-conceived. The notion that the executive branch could be immune from constitutional review is as baseless as it is dangerous. The Ninth Circuit panel cited case after case in which the Supreme Court had reviewed — and, in some cases, invalidated — executive branch actions on immigration and national security matters.

The ample precedent left the judges with no doubt: “(I)t is beyond question that the federal judiciary retains the authority to adjudicate constitutional challenges to executive action.”

This holding is important precisely because Trump has questioned that authority, in a manner no other president has done. Disagreeing with a judicial decision is the right of every litigant, including the president.

Trump, however, has publicly challenged the legitimacy of the judges issuing the decisions. In a blizzard of tweets, he referred to the trial judge as a “so-called judge.” He derided courts as “so political” — a bizarre response, considering that the trial judge and one of the panel judges were appointed by Republican presidents.

And he tried to cow them into retreat, threatening that blood would be on their hands.

"After appeals court ruling, Trump has two unappealing options" https://t.co/ibKsvkBkcv pic.twitter.com/A2QUonQi4X — The Hill (@thehill) February 10, 2017

Against this onslaught, the Ninth Circuit’s unapologetic defense of its constitutional role is heartening. After all, the legal battle over the travel ban is just beginning.

The same trial judge who temporarily stayed the order must now decide whether to continue the stay until he reaches his final decision in the case. This time, government lawyers will present evidence to justify the order, and there is sure to be disagreement over how strong that evidence is — and how strong it needs to be.

However the judge rules, the losing party will appeal, perhaps all the way to the Supreme Court. And all of that is before the trial court even gets to the “merits” phase — the proceedings that result in the trial court’s final ruling.

Moreover, the ban itself is only the first step envisioned by the executive order. Trump also charged the Secretary of Homeland Security with developing a new vetting system while the ban is in effect. Reports of how customs officials have been treating Muslims coming into the U.S., including American citizens, give some indication of what this vetting may entail.

Travelers have been interrogated about their religious beliefs, political views, and attitude toward the administration.

Their cell phones have allegedly been seized and copied, and they have been required to turn over their social media passwords — a requirement DHS Secretary John Kelly wants to make standard practice when foreign nationals seek to enter the U.S.

Such a system would raise the constitutional stakes to a level not seen since the McCarthy era. It is the courts that will ultimately decide these weighty constitutional issues, along with others implicated by actions Trump has either taken or proposed. That is their role in our constitutional system of checks and balances.

The Ninth Circuit’s decision is a powerful vindication of that role.

Elizabeth Goitein is co-director of the Liberty & National Security Program at the Brennan Center for Justice at NYU School of Law.

The views of contributors are their own and not the views of The Hill.