Donald Trump's homeland security secretary, Kirstjen Nielsen, dramatically quit her post on Sunday night, becoming the latest in a series of high-profile figures to leave his administration.
Ms Nielsen's departure was confirmed by the President on Twitter and came after she met Mr Trump at the White House.
It is understood Mr Trump was unhappy with her performance at Homeland Security as he moved towards taking a tougher line on immigration.
Only last week, speaking in Las Vegas, the US president declared that "our country is full" as he placed immigration firmly on the agenda of his re-election campaign.
According to reports in Washington, the final straw came when Ms Nielsen was blindsided by the White House decision to rescind the nomination of Ronald Vitello as head of Immigration and Customs, one of the most senior officials in her department.
Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service....— Donald J. Trump (@realDonaldTrump) April 7, 2019
Ms Nielsen's tenure since succeeding John Kelly as Homeland Security Secretary in December 2017 has been uncomfortable.
In May last year, she reportedly offered her resignation after she was upbraided over immigration by Mr Trump in front of cabinet colleagues, but was dissuaded from doing so by Mike Pence, the vice president.
The Department of Homeland Security denied the report in the New York Times, and Ms Nielsen then issued a statement in which she sought to play down the reported rift.
"The President is rightly frustrated that existing loopholes and the lack of Congressional action have prevented this administration from fully securing the border and protecting the American people," she said. "I share his frustration."
Describing border security as essential to the United States, she added: "I will continue to direct the department to do all we can to implement the president's security-focused agenda".
Within days she was appearing before Congress defending the controversial separation of immigrant families at the border.
Then, in November, she came under vitriolic attack from the party in opposition. "You are responsible for the unimaginable trauma of thousands of children across the United States," senior Democrats told her in a letter.
"From the children torn from their parents at the border, to Dreamers facing exile from their home, and to the US citizen children who face losing a parent to deportation; your actions continue to wreak havoc on communities across this nation."
The following month the crisis intensified when Jakelin Caal, a seven-year-old girl from Guatemala, died in the custody of US Customs and Border Protection, one of her agencies.
Her position had been weakened by the departure of John Kelly as chief of staff, who had been one of her strongest allies.
Ms Nielsen had not only aroused the ire of Mr Trump, but also Stephen Miller, the president's hardline immigration adviser.
They felt that she had failed to protect the US border and held her responsible for the sharp increase in migrants trying to enter the country from Latin America.
Ms Nielsen will be replaced, for the time being, by Kevin McAleenan, the commissioner of Customs and Border Protection.
Her departure is a further reflection of the revolving door in Mr Trump's administration which has seen at least 40 high-profile departures since he took office.
Departures include a raft of cabinet members such as Rex Tillerson, the Secretary of State, Ryan Zinke, the Interior Secretary, Tom Price, the Health and Human Services Secretary, and James Mattis, the Defence Secretary.
Ms Nielsen's resignation was seized upon by John Delaney, one of the Democratic candidates for the 2020 presidential nomination.
“The fact that Secretary Nielsen can no longer be an effective messenger for the White House shows just how out of touch and cruel the President’s immigration policies are," he said. "Congress should do its due diligence and properly vet the next DHS Secretary."
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