Homeland Security Secretary Kirstjen Nielsen resigned on Sunday amid President Donald Trump’s growing frustration and bitterness over the number of Central American families crossing the southern border.

Trump announced on Sunday in a tweet that US Customs and Border Protection Commissioner Kevin McAleenan would be taking over as acting head of the department. McAleenan is a longtime border official who is well-respected by members of Congress and within the administration. The decision to name a top immigration officer to the post reflects Trump’s priority for the sprawling department founded to combat terrorism following the Sept.11 attacks.

“I have determined that it is the right time for me to step aside,” Nielsen wrote in her resignation letter. “I hope that the next secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America’s borders and which have contributed to discord in our nation’s discourse.” Though Trump aides were eyeing a staff shake-up at Homeland Security and had already withdrawn the nomination for another key immigration post, the development Sunday was unexpected.

Nielsen travelled to the US-Mexico border on Friday with Trump to participate in a roundtable with border officers and local law enforcement. There she echoed Trump’s comments on the situation at the border, though she ducked out of the room without explanation for some time while Trump spoke. As they toured a section of newly rebuilt barriers, Nielsen was at Trump’s side, introducing him to local officials. She returned to Washington afterward on a Coast Guard Gulfstream, as Trump continued on a fundraising trip to California and Nevada.

Nielsen had grown increasingly frustrated by what she saw as a lack of support from other departments and increased meddling by Trump aides on difficult immigration issues, according to three people familiar with details of her resignation. They spoke on condition of anonymity because they were not authorised to discuss the matter.

She went into the White House on Sunday to meet with Trump not knowing whether she’d be fired or would resign. She ended up resigning, though she was not forced to do so, they said.

Nielsen is the latest person felled in the Trump administration’s unprecedented churn of top staff and Cabinet officials, brought about by the president’s mercurial management style, insistence on blind loyalty and rash policy announcements.

Nielsen was also the highest profile female Cabinet member, and her exit leaves DHS along with the Pentagon and the White House staff itself without permanent heads. Patrick Shanahan has held the post of acting defense secretary since the former secretary, Jim Mattis, was pushed out in December over criticism of the president’s Syria withdrawal plans. Acting White House chief of staff Mick Mulvaney has held his post since January, following John Kelly’s resignation last year.

Her resignation lett er lacked any sense of controversy — unlike those of others who have left. She thanked Trump and DHS staff for their hard work.

Her replacement, McAleenan, has helped shape many of the administration’s policies to date and is considered highly competent by congressional leaders, the White House and Homeland Security officials. But it’s unclear if he can have much more of an effect on the issues at the border. The Trump administration has bumped up against legal restrictions and court rulings that have hamstrung many of its major efforts to remake border security.

Tensions between the White House and Nielsen have persisted almost from the moment she became secretary, after her predecessor, Kelly, became the White House chief of staff in 2017. Nielsen was viewed as resistant to some of the harshest immigration measures supported by the president and his aides, particularly senior adviser Stephen Miller, both on matters around the border and others like protected status for some refugees.

Once Kelly left the White House, Nielsen’s days appeared to be numbered. She had expected to be pushed out last November, but her exit never materialized. During the government shutdown over Trump’s insistence for funding for a border wall, Nielsen’s standing inside the White House even appeared to rise.

Associated Press