Nielsen served under President George W. Bush in the Transportation Security Administration and on the Homeland Security Council, where she oversaw disaster preparedness and response.
She didn't then, and went on to become one of the public faces of a controversial policy that separated undocumented children from their parents.
After CBS News reported Nielsen meant to resign Sunday from the position she has held since December 2017, the president confirmed on Twitter she would be leaving her position.
The president said on Sunday Kevin McAleenan, the current US Customs and Border Protection commissioner, would become the acting DHS secretary. The Wall Street Journal reported in November that Trump had chose to replace Ms Nielsen.
Trump announced that Nielsen was leaving the department in a tweet on Sunday, in which he briefly thanked her for her service.
"System has been broken for many years". He is left trying to disparage their plight (e.g. calling them terrorists, asserting that their claims of persecution are phony) or make ridiculous promises (e.g. close the border). The change at the top comes with other uncertainty over management at the Department of Homeland Security.
Now this President wants someone even "tougher". Trump cut aid to the three countries last month in response to the surge.
"An extension of colonialist theory, it is predicated on the notion that white women are not having enough children and that falling birthrates will lead to white people around the world being replaced by nonwhite people", explained reporter Nellie Bowles in the New York Times after the New Zealand shooting.
The announcement on Twitter came shortly after Trump and Nielsen met at the White House, according to two senior administration officials.
Nielsen, 46, held the position since December 2017. And what he brings that's unique is over a decade of specific border security experience.
Images of sobbing children being taken from their parents a year ago fuelled a national outcry as condemnation poured in from the United Nations, human rights groups, and four former first ladies - all mothers - who called the policy "cruel" and "immoral". And Miller's views, echoed and lauded by pundits like Fox Business Network's Lou Dobbs, have inspired repeated and sustained right-wing criticism of Nielsen throughout her tenure. Trump announced Nielsen's resignation on Sunday. 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.
The president railed against immigration last week, saying that the "country is full", and demanding that Mexico do more to curb illegal immigration.
"I did not hear that word used", she told a congressional hearing.
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