We've been expecting it

Pleased because men kiss ass better than women, and because he now has another "acting" cabinet member, which he prefers because he can ignore them more easily.

The U.S. has acting secretaries of defense and interior, attorney general, White House chief of staff, and heads of the Environmental Protection Agency, Office of Management and Budget, National Park Service, Bureau of Land Management, and other major federal agencies. For President Trump, this is a feature not a bug, he told CBS's Margaret Brennan in his pre-Super Bowl interview. "It's easier to make moves when they're acting," he said. "Really, I like acting because I can move so quickly. It gives me more flexibility."

  The Week - February 4, 2019

Kirstjen Nielsen, the homeland security secretary who has been the public face of some of the Trump administration’s most contentious policies, has resigned.

[...]

Nielsen’s position had been rocky for some time. In November, leaks from the White House suggested she would be out by the end of the year as Trump fulminated against what he complained was her weak performance.

The previous May, the New York Times reported that Trump had humiliated her in front of the entire cabinet, castigating her for failing to reduce the numbers of undocumented migrants entering the US from Mexico. The newspaper said she considered quitting then.

[...]

In office, Nielsen had to defend several of Trump’s most controversial policies, including his increasingly eccentric demands for a border wall, which caused a 35-day government shutdown, his declaration of a national emergency to access funds for the wall, and last summer’s family separations at the border which led to accusations of cruelty around the world.

  Guardian

And each time she got grilled by one committee or another, she didn't seem to have a clue regarding numbers and regulations. Or pretty much anything.

Ronald Newman, political director of the American Civil Liberties Union, charged Nielsen with allowing DHS to act as the “key cog in Trump’s unconstitutional and anti-immigrant agenda of fear”. He accused her of overseeing “gross civil rights and civil liberties violations that will go down as among the worst failures in Trump’s chaotic presidency”.

[...]

On Friday, Nielsen accompanied Trump to the border.

Apparently something happened then that was the last straw. Whether she resigned on her own account or was told to resign is unspoken.

According to CBS News, which first reported the news of Nielsen’s impending resignation, her departure is yet another victory for Stephen Miller, the far-right senior adviser in the White House who has consistently steered the president in a hardline direction on immigration.

The extreme hand of Miller was also seen behind Trump’s sudden announcement on Friday that he had withdrawn his nominee as new director of US Immigration and Customs Enforcement (Ice). Trump said he was pulling Ronald Vitiello as “we want to go in a tougher direction”.

[...]

Seventeen senior administration figures have departed so far and the key roles of secretary of defense, secretary of the interior, homeland security secretary, White House chief of staff and ambassador to the United Nations are currently filled in an acting capacity.

[...]

The removal of Nielsen further skews the composition of Trump’s cabinet in a hyper-male direction, with the resignation of Linda McMahon, announced in March, to take effect this week. Of the cabinet members remaining, only three are women: Gina Haspel as CIA director; Betsy DeVos as education secretary; and Elaine Chao in charge of transportation.

Those three can stay. Gina Haspel because she's an experienced and willing torturer; Betsy DeVos because she's trying to destroy public education, something Trump cares nothing about; and Elaine Chao because she's Mitch McConnell's wife.

It emerged on Saturday that the Trump administration has predicted it could take two years to identify all families affected by the separations policy. Nielsen famously and repeatedly claimed no such policy existed.

In her resignation letter, Nielsen continued to press the line that she had adopted in post over several months: that Congress and federal judges, not Trump, were responsible for the rising mayhem at the border.

She wrote: “Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside.

Needs to spend more time with her family.

...but hey, do what you want...you will anyway.

UPDATE:






Impeach.

UUPDATE: It appears Trump is not, according to statue, allowed to put his choice into Nielsen's place.  DHS has rules for succession.

UPDATE:  Yeah, she was told to resign.