NAYIB BUKELE delivered what looked like a healthy jolt to El Salvador’s political system when he was elected president a year ago. The populist former mayor of San Salvador crushed the two parties that have dominated politics since the country’s bloody civil war, one representing the right and the other the former insurgent left. Neither had proved able to stem the official corruption and runaway gang violence that have plagued the country for decades.

Mr. Bukele’s popularity has soared since taking office last June: One December survey had his approval rating at 88 percent. That’s largely because violence, at least by the government’s measure, has plummeted. The number of recorded homicides fell from 285 last May to 119 in January, the lowest figure since the peace accord of 1992. Though experts differ on the reasons, Mr. Bukele says the plunge reflects the success of his Territorial Control Plan, which has deployed more security forces against the gangs and tightened the controls over their imprisoned leaders.

So far, so good. Only now Mr. Bukele, at 38 the youngest president in the country’s history, has appeared to let his success go to his head. On Sunday, he delivered another jolt to the political establishment, leading scores of heavily armed troops and police into a meeting of the Legislative Assembly. He acted after legislators balked at his demand for immediate approval of a $109 million loan to buy more supplies for the counter-gang campaign. Saying, “I think it’s very clear who has control of the situation,” the president said he was giving the assembly a week to pass the enabling legislation.

Though it may not have been a “coup,” as opposition leaders described it, the president’s military-backed intimidation was an alarming violation of democratic norms, especially in a country where a politicized military helped trigger a 12-year war in which more than 75,000 people died. Fortunately, the stunt generated quick and broad pushback. The Supreme Court on Monday rebuked both the president and the army, ordering the latter to stay out of politics; newspapers published scathing editorials; and opposition leaders said they would not submit to Mr. Bukele’s threats.

Constructive criticism also came from the U.S. ambassador in El Salvador, Ronald Johnson, who tweeted his disapproval (in Spanish) of the military intervention and added that “El Salvador will only maintain the positive advances in security and promote economic growth if all arms of government work independently, respect the rule of law, maintain the apolitical role of the armed forces and national police and promote stability.” Mr. Bukele has enjoyed warm relations with the Trump administration, with which he struck deals on migrants and Salvadoran workers in the United States. He needs to hear consistently that Washington will not accept the casual destruction of a democracy it spent years helping to build.

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