Homeland Security Secretary Kirstjen Nielsen speaks during a press briefing on border security at the White House in Washington, April 4, 2018. (Photo: Reuters)

Kevin McAleenan, current Customs and Border Protection Commissioner, will become Acting Secretary. Nielsen has served as Homeland Security chief since December, 2017. She was responsible for the proposed border wall with Mexico and the separation of migrant families. Her resignation came after the President indicated that he wants a tougher immigration policy. Trump has often accused Ms. Nielsen of not being tough enough.