Tom Bossert, White House homeland security advisor, is resigning, the White House confirmed Tuesday.

Context: Bossert's departure isn't the biggest one to hit the White House in recent weeks, but it adds to the accumulated momentum of exits, sense of turmoil, and transition. This is an especially uncertain moment for a White House in flux on multiple counts: policy, personnel, and Trump’s own mood towards the Russia investigation. The president is now seriously flirting with firing Special Counsel Robert Mueller and every new change adds to anxiety levels among current staff.

The White House statement: “The President is grateful for Tom’s commitment to the safety and security of our great country," said Press Secretary Sarah Sanders. "President Trump thanks him for his patriotic service and wishes him well.”

The backstory, per Axios' Jonathan Swan: Senior staff were initially worried about former National Security Advisor Mike Flynn and thought somebody was needed to "keep an eye on him." Bossert was partly brought in to fulfill that role and focus more on domestic terrorist threats — but it never worked out that way as Flynn was gone in a matter of weeks. H.R. McMaster's entrance further essentially stripped Bossert of his terrorism work. Bossert ended up playing an important role on natural disasters, but that was a very different job than what he was brought in to do.

Bossert also didn't have much of a relationship with the president until last summer's hurricanes — on which he briefed Trump. POTUS thought he did a good job and, for the most part, was good on TV, but Bossert wasn't a staffer who spent a lot of time in the Oval Office.

A source who's worked with Bossert in the White House tells Swan: "His primary role ended up being the point person responding to the natural disasters, of which there were many. He invested a lot of time, effort and energy in that."