TEN DAYS after walking in combat fatigues through Lafayette Square with President Trump, just after the park was cleared of peaceful protesters, the chairman of the Joint Chiefs of Staff, Army Gen. Mark A. Milley, has apologized for getting involved in domestic politics. The general is on target. He should never have been there, and it is to his great credit to admit the mistake. It may encourage others always to respect the Constitution, even when a president is beckoning to cross the line.

On June 1, people protesting the killing of George Floyd in police custody were pushed back with the use of chemical irritants, and Mr. Trump walked to a photo opportunity to hold up a Bible at St. John’s Episcopal Church, trailed by Gen. Milley, Defense Secretary Mark T. Esper and Attorney General William P. Barr, among others. Earlier, Mr. Esper had joined the president in a conference call with governors and promised federal support against violence. Mr. Esper told the governors, “I think the sooner that you mass and dominate the battlespace, the quicker this dissipates and we can get back to the right normal. We need to dominate the battlespace.”

Both Mr. Esper and Gen. Milley showed miserable judgment. The military’s purpose is to fight foreign adversaries, the United States is not a “battlespace,” and walking along with Mr. Trump in a park where peaceful demonstrators had been repulsed sent a terrible signal. Last week, both the secretary and the chairman, having come under blistering criticism, issued letters to the troops reaffirming their commitment to the oath all have taken to uphold the Constitution and its values of free speech and assembly.

Mr. Trump has surrounded himself with sycophants who celebrate his offenses or people who lack the courage to speak up — sometimes even after they have left public service. Gen. Milley’s forthright apology Thursday is a welcome departure from that norm. “I should not have been there,” he said. “My presence in that moment, and in that environment, created the perception of the military involved in domestic politics.” Gen. Milley said it “was a mistake that I have learned from.” Both Mr. Esper and Gen. Milley have suggested they were not clear that the stroll was going to end in a political photo op. Both should have learned by now that Mr. Trump smashes through norms and principles with reckless abandon, taking his subordinates with him.

Behind the scenes, Gen. Milley is reported to have discouraged Mr. Trump from invoking the Insurrection Act of 1807 to bring active-duty troops to the streets, and Mr. Esper announced that such a move is a last resort, not called for now. This triggered Mr. Trump’s fury. The enduring lesson is that the nation’s principles are stronger and larger than Mr. Trump’s greed for approbation. Gen. Milley’s statement is an important affirmation of that point.

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