Department of Homeland Security Secretary Kirstjen Nielsen resigns

Department of Homeland Security Secretary Kirstjen Nielsen submitted her resignation to President Donald Trump Sunday.

"Its been an honor of a lifetime to serve with the brave men and women of @DHSgov. I could not be prouder of and more humbled by their service, dedication, and commitment to keep our country safe from all threats and hazards," Nielsen posted on Twitter, along with a copy of her resignation.

Nielsen will stay until April 10 "to assist with an orderly transition and ensure that key DHS missions are not impacted," she stated on Twitter.

Kevin McAleenan, the current U.S. Customs and Border Protection commissioner, will become the acting secretary for DHS, Trump announced on Twitter.

Nielsen was scheduled to meet with the president at 5 p.m. in the White House residence about border security, but sources told ABC News that the meeting turned into a discussion about Nielsen's future.

On her way back to Washington, D.C., from the border Friday, Nielsen called Trump asking to meet over the weekend about a path forward with regard to the border, a source close to Nielsen told ABC News.

The president had been refusing to believe Nielsen couldn't make the worsening border numbers go down herself and had been holding her personally responsible for the numbers, the source said.

This "led to intense clashes, angry phone calls, and frustration between her and the president," the source said.

A veteran of the George W. Bush administration, Nielsen had been chief of staff to then-Homeland Security secretary John Kelly before he became Trump's chief of staff at the White House.

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.

At the helm of DHS, Nielsen was a major player in implementing the administration’s highly controversial “zero-tolerance” policy that resulted in the separation of thousands of migrant children from their parents during the summer of 2018.

Nielsen became the public face of the policy, and at one point, was targeted by protesters who heckled from a few feet away while she dined at a Mexican restaurant in Washington, D.C.

"To a select few in the media, Congress and the advocacy community, I'd like to start with a message for you: this department will no longer stand by and watch you attack law enforcement for enforcing the laws passed by Congress," Nielsen said in June 2018, defending the policy while speaking to the National Sheriffs' Association in New Orleans.

Known as a cybersecurity policy wonk, Nielsen saw Homeland Security through a successful 2018 midterm elections amid concerns over cyber attacks. She had repeatedly said Russia was to blame for meddling in the 2016 presidential elections while not explicitly supporting special counsel Robert Mueller's investigation into Russian interference.

Trump nominated Nielsen to be DHS secretary in October 2017. She was sworn in as DHS secretary in December 2017.

Democrats and Republicans had differing reactions to Nielsen's resignation.

"It is deeply alarming that the Trump Administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House's liking," House Speaker Nancy Pelosi said in a statement.

House Homeland Security ranking member Rep. Mike Rogers, R-Ala., said Nielsen "served her country honorably as Homeland Security Secretary, despite facing numerous challenges including dire conditions at our southwest border."