Behind the scenes: Kirstjen Nielsen sacked by tweet

"She was undercut at every turn," a source close to DHS said. "She's done everything she can do. The White House is eating their own."

Between the lines: Nielsen had been on the outs with some in the West Wing for at least six months, top officials tell us.

• National security adviser John Bolton has felt the increase in immigration numbers made it clear that her policies weren't effective, and he thought the president should relieve her of her duties, a senior administration official said.
• Last fall, Bolton took his advice about Nielsen to Trump, incurring the wrath of then-chief of staff John Kelly, a Nielsen protector.

• Back in October, accounts surfaced of a shouting match between Bolton and Kelly. It turns out that it was over Bolton's Nielsen conversation with Trump.

Be smart: Nielsen's departure empowers White House hardliner Stephen Miller.

• A Republican Senate aide tells Axios: “Nielsen leaving will make conservatives who were getting fed up with DHS happy."
• "Real question will be who’s the [permanent] replacement and does that person have the credentials?"
• "Whoever replaces will have one hell of a confirmation hearing.”

Go deeper: Read the resignation letter