Homeland Security Secretary Kirstjen Nielsen leaving

Homeland Security Secretary Kirstjen Nielsen submitted her resignation to President Donald Trump Sunday night, in an unexpected move that appears related to the president’s ongoing rage over the number of Central American families and asylum-seekers coming into the United States.

Kevin McAleenan, the head of Customs and Border Protection, will serve as acting DHS secretary. It’s not yet clear whether Trump will formally nominate a successor to Nielsen in the near future.

Nielsen, the sixth head of the Department of Homeland Security and Trump’s second appointee (she was confirmed in December 2017 after mentor John Kelly left to become White House chief of staff), has arguably been the most aggressive secretary in the department’s short history in cracking down on immigration — with her legacy likely to be defined among progressives by the “zero tolerance” prosecution policy of late spring and early summer 2018 that resulted in the separation of thousands of families at the US/Mexico border.

None of it appears to have been enough for Donald Trump.

Nielsen’s resignation was preceded on Thursday night by the abrupt withdrawal of the nomination of acting Immigration and Customs Enforcement director Ron Vitiello to formally lead the agency, with Trump telling reporters Friday morning that he wanted to go in a “tougher direction.” While it’s not yet clear whether Trump requested Nielsen’s resignation or not, it certainly appears as if that “tougher direction” is extending to a new DHS secretary. (Earlier in the week, it was reported that Trump was considering a White House-based “immigration czar” to coordinate various immigration-related agencies — most of which are under the aegis of DHS.)