The decision, which Trump announced on Twitter, comes just two days after the president abruptly pulled back his nominee to lead Immigration and Customs Enforcement because he said he wants “to go in a tougher direction.” The moves signal that Trump is seeking to shake up his team amid frustration over the spike in migrant families crossing the southern border.

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Trump tweeted that Kevin McAleenan, the commissioner of Customs and Border Protection, will lead the Department of Homeland Security (DHS) on an acting basis until a permanent replacement for Nielsen is chosen.

....I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSgov. I have confidence that Kevin will do a great job! — Donald J. Trump (@realDonaldTrump) April 7, 2019

In her resignation letter , Nielsen wrote that she had “determined that it is the right time for me to step aside.”

“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,” she wrote.

The president thanked Nielsen for her service and said he has “confidence that Kevin will do a great job!” But he did not explain what led to her exit.

John Kelly John Francis KellyMORE Nielsen led the sprawling department’s efforts on immigration enforcement, disaster relief, election security and cybersecurity since December 2017. She took over for her allyas the head of DHS after he vacated the role to serve as White House chief of staff. Kelly left the White House at the end of 2018.

During her tenure, Nielsen dealt with persistent rumors that her job was in jeopardy as the president often seethed about the increase in migrant families and unaccompanied minors entering the U.S. and faulted her for not fixing the problem.

ADVERTISEMENT Trump last November was set to fire Nielsen, who had privately expressed frustration about dealing with the president’s anger over the situation at the border. But her job security once again became tenuous after Kelly’s exit last December. Kelly was a mentor to Nielsen and defended her against attacks from Trump and White House policy adviser Stephen Miller, an outspoken immigration hawk, according to reports.

The White House did not immediately respond to a request to explain Nielsen’s departure, but it reportedly came after she met with Trump on Sunday in the White House residence.

Nielsen also came under fire from Democrats for her role in defending the administration’s immigration agenda, which they have criticized as cruel to migrant families.

Earlier this year, Nielsen became the public face of the administration’s controversial “zero tolerance” policy that led to the separation of thousands of migrant families that illegally crossed the southern border.

“It is clearer now than ever that the Trump administration's border security and immigration policies - that she enacted and helped craft - have been an abysmal failure and have helped create the humanitarian crisis at the border,” Thompson said. “It is truly unfortunate that Nielsen refused to take responsibility for her actions and was simply unable to lead and stand up to the president for his misguided, wall obsessed anti-immigrant agenda.”

Nielsen stood at the podium in the White House briefing room in June and insisted the administration did not have a policy of separating families and that only Congress could address the underlying issue. Within days of her remarks, Trump signed an executive order ending the practice.

She also was a vocal defender of Trump's controversial decision to declare a national emergency to circumvent Congress and obtain funding for a border wall. Nielsen cut short an overseas trip this week to spend three days along the border, even making an appearance with Trump at a newly renovated section of border fencing in Calexico, Calif.

While she earned praise from the president for her performances, Nielsen could not escape Trump’s scorn as he sought to implement stricter immigration laws. She also reportedly considered resigning in May after he lashed out at her in a lengthy tirade during a Cabinet meeting.

Prior to her time in Trump’s Cabinet, Nielsen served in the George W. Bush administration in the Transportation Security Administration and on the Homeland Security Council.

Nielsen also worked in the private sector and academia with a focus on cybersecurity before she returned to the federal government to work as Kelly's deputy both at DHS and at the White House.

Her departure will be viewed as a blow to DHS's cyber policies and operations, which she often championed and publicly discussed. The site CyberScoop reported last year that Nielsen had been offered the top cyber job at the department but that she was unable to decide whether to take up the post.

Nielsen was also a vocal advocate for improved election security as DHS sought to help state and local officials secure their elections after Russia interfered in the 2016 race.