William Barr was attorney general from 1991 to 1993.

On Friday, President Trump issued an executiveorder temporarily suspending for 90 days the entry of nationals from seven countries into the United States pending the implementation of heightened vetting procedures to identify and exclude any radical Islamist terrorists attempting to infiltrate the country. Like many others, I thought part of that order — the manner in which it was applied to permanent residents — though not illegal, was unwise, but that aspect has been remedied. I see no plausible grounds for disputing the order’s lawfulness. It falls squarely within both the president’s constitutional authority and his explicit statutory immigration powers. Nonetheless, over the past several days, the left, aided by an onslaught of tendentious media reporting, has engaged in a campaign of histrionics unjustified by the measured steps taken.

On Monday, things reached their nadir when acting attorney general Sally Yates, an Obama holdover with a few days left in office, issued a directive that the Justice Department should not defend the president’s order in court. While an official is always free to resign if she does not agree with, or has doubts about, the legality of a presidential order, Yates had no authority and no conceivable justification for directing the department’s lawyers not to advocate the president’s position in court. Her action was unprecedented and must go down as a serious abuse of office.

In our system of government, the Supreme Court ultimately decides on the constitutionality of laws passed by Congress or ofpresidential actions. When their actions are challenged, both Congress and the president are entitled to have their positions forcefully advocated in court. It is the responsibility of the Justice Department to be that advocate. That is why the department has long recognized that, even if it doubts the legality of a statute, it is obliged to defend that law by advancing all colorable arguments that can mustered in support. And when the president determines an action is within his authority — even if that conclusion is debatable (which I don’t think it is here) — the president is entitled to have his position presented to the courts. It is the duty of the department to present them.

Yates’sattempt to justify her action is incoherent and untenable. The crux of her position was not that the order was illegal but that its legality is open to dispute and she had yet to be convinced it was legal. Indeed, she acknowledged that the department’s own Office of Legal Counsel had concluded that the order was legal. Instead, she vaguely suggested that the president could have hidden motives for the order that somehow affect its legality. She never explained what these are or how they could invalidate the order. She summarily justified her obstruction on the grounds that she was not yet “convinced that the Executive Order is lawful” and that she did not think it wise policy. While she was free to resign if she disagreed, neither her policy objection nor her legal skepticism can justify her attempt at overruling the president.

Presidential powers are not exercised by a body or group. The Constitution vests “all executive power” in one and only one person — the president. An attorney general’s duty is to render her opinion and honest advice; she cannot set herself up as a judge overruling the president’s decision. The president need not “convince” his subordinate that his decision reflects the best view of the law.

The absurdity of Yates’s position is self-evident. If it is permissible for her, based on her own opinion, to direct the president’s subordinates not to carry out or defend a presidential directive, then it would be permissible for her own subordinates to do the same to her. If she, as acting attorney general, decided that a particular case should be brought, would it be permissible for any official down the chain to flout and sabotage her decision by directing their own subordinates to defy her? No government could function in that way.

By her vague reference to the president’s possible hidden motivations, Yates was attempting to advance the narrative that the vetting order, though cast as a national security measure, is really a discriminatory Muslim ban. The very terms of the order expose this claim as baseless. First, of the 49 majority-Muslim countries in the world, the 90-day suspension applies only to seven, comprising about 12 percent of the world’s Muslim population. Second, it is clear that the criterion for selecting those seven countries was not that they were Muslim but that the risk of terrorist infiltration from these countries is especially high. Third, the order merely suspends entry while a vetting process is implemented. By definition, a vetting process means that exclusion will not be based on attributes such as religion, but on the attributes detected through vetting — namely, the violent, hostile ideology that Islamist militants possess. Nor does the indefinite suspension of refugees from Syria suggest anti-Muslim animus. That measure makes perfect sense given the president’s plan to establish “safe zones” that will protect innocent civilians inside Syria.

Trump could not allow Yates’s obstruction to stand. To have allowed it would have set a dangerous precedent.