While she was racing to put out the letter (not that different from one she wrote after midterms), Trump tweeted that she "will be leaving her position". She appeared to recover her footing after U.S. Border Patrol agents used tear gas to repel a large crowd attempting to break through the border fence - the kind of "tough" action Trump said he wanted in a DHS secretary.
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.
Nielsen served as DHS secretary during a tumultuous period for the department, especially as it related to her oversight of Immigration and Customs Enforcement (ICE) and Customs and Border Protection (CBP) throughout President Donald Trump's quest to clamp down on illegal immigration and build a wall along the southern border.
Unsurprisingly, White House senior adviser Stephen Miller is reportedly behind the shake-up at DHS. But none of it was enough for Trump, who clearly wants an even more aggressive approach to white-ifying the United States and demanded Nielsen's resignation over the weekend.
The president recently vowed to cut all US foreign aid to Guatemala, Honduras and El Salvador and has repeatedly threatened to close ports of entry to and from Mexico if the country's government does not do more to prevent migrants from the Northern Triangle from reaching the southern border.
Trump threatens to put tariffs on cars coming from Mexico
Pelosi said in a statement Thursday that Trump was "stealing from appropriated funds". "I'll do it", he said. The U.S. and Mexico trade about $1.7 billion in goods daily, according to the U.S.
The departure announcement comes just days after Nielsen visited the U.S. -Mexico border in Calexio, California, with Trump.
During her tenure, Nielsen also faced scrutiny for comments she made regarding Russian interference in the 2016 presidential election and beyond. But she expressed some frustration with the current immigration system. The people say she has always been frustrated by the difficulty getting other departments to help with the growing number of families coming crossing the border.
La secretaria de Seguridad Nacional, Kirstjen Nielsen, en el escenario para hablar se sube al escenario mientras es presentada para hablar en la convención de la Asociación Nacional de Jefes de Policía en Nueva Orleans, el lunes 18 de junio de 2018.
"I did not hear that word used", she told a congressional hearing.
"Norway is a predominantly white country, isn't it?" a senator asked her. Moreover, the asylum crisis is not a border security issue since these people are presenting themselves at the border.