Opinion | Kirstjen Nielsen Enforced Cruelty at the Border. Her Replacement Could Be Worse.

Time finally ran out for Kirstjen Nielsen, President Trump’s beleaguered secretary of homeland security.

The terms of Ms. Nielsen’s departure were unclear. She met with the president on Sunday evening to discuss continuing problems at the southern border. At the conclusion of the meeting, Mr. Trump said on Twitter that Ms. Nielsen “will be leaving her position” and thanked her for her service, implying he had asked her to step down. Ms. Nielsen issued a formal letter of resignation, saying it was the “right time for me to step aside.” Considering the long-simmering tensions between the president and Ms. Nielsen, the most surprising thing about her departure may be that it didn’t happen months ago.

She was said to have become increasingly insecure in her job in recent weeks, as Mr. Trump repeatedly railed about the chaos at the border and vowed to move in a “tougher” direction. The president grew impatient with Ms. Nielsen’s insistence that federal law and international obligations limited her actions.

It’s no secret that Mr. Trump had a problem with Ms. Nielsen, whom he considered “weak” on matters of border security. The president and Stephen Miller, his hard-line immigration adviser, have long grumbled privately about the secretary’s insufficiently brutal approach to the surge in migrant families across the border. Last May, stories surfaced about Mr. Trump publicly berating her in front of the entire cabinet for failing to stop the crossings. Ms. Nielsen was said to have drafted a resignation letter at the time.

In her resignation letter released on Sunday, Ms. Nielsen noted, “For more than two years of service beginning during the Presidential Transition, I have worked tirelessly to advance the goals and missions of the Department.”

This is hardly something to brag about. Whatever the secretary’s personal views, and no matter how impossible her job, she was the face of some of the administration’s most poorly conceived and gratuitously callous policies. At best, she was complicit and, yes, hopelessly weak.

Sadly, Ms. Nielsen’s response to her boss’s displeasure and abuse was both morally anemic and strategically incoherent. Last summer, as Republicans and Democrats — and many in the American public — protested the administration’s practice of tearing migrant children from their parents at the border, Ms. Nielsen rushed to publicly defend the policy. Scratch that. She insisted, repeatedly and bizarrely, that the administration had no such policy, even as her agency was enforcing and justifying it.

“We do not have a policy of separating families at the border,” she said on Twitter last June. “Period.” She repeated as much to Congress as recently as March.

Nor was immigration the only issue on which Ms. Nielsen floundered. On the critical question of Russia’s meddling in the 2016 elections, she was even less lucid. At times, she seemed to support the intelligence community’s findings that the Kremlin had been up to no good. Other times, she supported the view that Russia had not favored Mr. Trump in the election. Her every utterance seemed designed to obfuscate rather than clarify.

Ms. Nielsen’s departure is seen by some as part of a broader restructuring of her department. Just two days before meeting with the secretary, the president withdrew his nomination for the next head of the Immigration and Customs Enforcement agency, saying that he wanted to go in a “tougher direction.” Presumably he plans to chart a similar course with Ms. Nielsen’s successor.

For now, Ms. Nielsen’s acting replacement will be Kevin McAleenan, the commissioner of Customs and Border Protection. This leaves Homeland Security without a top official at either of its critical immigration agencies. It comes as the swell of migrant families across the border pushes the system toward collapse.

Within this leadership vacuum, it seems likely that more influence will be exerted by Mr. Miller, who inspires and reinforces Mr. Trump’s harshest ideas on immigrants and immigration.

The House speaker, Nancy Pelosi, said of Ms. Nielsen’s departure, “It is deeply alarming that the Trump administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House’s liking.”

If Ms. Nielsen wants to perform one last act of public service, she could come clean about the costs of the policies she enforced over the past year and half, not only to the desperate migrants seeking a better life in the United States, but also to the thousands of employees of her department charged with carrying out an inhumane and ineffective set of policies.