| The New York Times

Kevin McAleenan Is Out as Acting Homeland Security Secretary

“I also believe strongly in investing in the growing collaboration with the governments of Guatemala, El Salvador and Honduras, and see real opportunity to change the dynamic in the region through these partnerships,” Mr. McAleenan said in his resignation letter.

The Vacancies Act stipulates that the position of secretary must go to certain ranked officials in the department, but according to two people briefed on the circumstances surrounding Ms. Nielsen’s departure, rules were changed that rendered the act moot, which may necessitate additional changes for the White House to appoint a person of the president’s choosing.

While Mr. McAleenan had the strong backing of Jared Kushner, the president’s son-in-law and senior adviser, he clashed over personnel decisions with Stephen Miller, a White House aide and the architect of Mr. Trump’s immigration agenda. In recent months, Mr. McAleenan grew increasingly irritated by the harsh language used by agency officials installed by the White House.

Kenneth T. Cuccinelli II, who once advocated an end to birthright citizenship, was installed to lead the agency that manages legal immigration. Mark Morgan, who once said he could determine future gang members by looking at detained migrant children, was selected to oversee Immigration and Customs Enforcement. He then replaced John Sanders, an ally of Mr. McAleenan, to lead Customs and Border Protection. Both are serving in acting positions.

Bennie Thompson, the chairman of the House Homeland Security committee, derided the department’s turnover. “President Trump cannot continue to rely on instilling ‘acting’ — and potentially unlawful — figureheads for this critical position,” Mr. Thompson said in a statement. “The next secretary must also understand that bowing to President Trump’s obsession over a wall and keeping people out is not part of the job description.”

Mike Rogers, Republican of Alabama and the ranking member of the committee, praised Mr. McAleenan for reducing the flow of migrant families to the border.

“Kevin McAleenan was dealt an incredibly tough hand when he took on the role of acting secretary this spring — we were in the midst of a humanitarian and security crisis at our southwest border,” Mr. Rogers said in a statement. “Fortunately, he was up to the task.”