OPINION: Kirstjen Nielsen Enforced Cruelty at the Border. Her Replacement Could Be Worse.

She will be remembered for the forced separation of thousands of migrant families.

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.”