Nielsen advocated for strong cybersecurity defence and often said she believed the next major terror attack would occur online - not by planes or bombs. 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 around the border and on other matters like protected status for some refugees.
Two sources said Nielsen went to the White House Sunday to speak with Trump following their trip to the border late last week.
Nielsen isn't the only administration official impacted by the apparent overhaul: On Friday, Trump announced he was pulling his nominee to lead U.S. Immigration and Customs Enforcement, the Associated Press reported.
She joined the administration as chief of staff to John Kelly, who was Trump's first Homeland Security secretary.More news: Cristiano Ronaldo: Why Zinedine Zidane Is Perfect For Real Madrid
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In recent weeks, administration officials have been sounding the alarm over the increase in migrants at the southern border, underscoring the change in demographics as one of the reasons for the host of challenges they're facing.
Nielsen was a close ally and confidante of Kelly, who left the administration at the end of 2018, marking a dramatic shift in the power dynamics of the West Wing, and there was continuing tension between Trump and Kelly over how the president treated her.
"We're going in a little different direction". But she was also instrumental in ending the policy.
Nielsen, 46, is among the longest-serving Cabinet heads in the Trump administration, and leads the federal government's third-biggest department, the Los Angeles Times reported last week in an article on how Nielsen had - for 15 months - "succeeded in holding onto her job, defying numerous reports that Trump had made a decision to fire her".