Behind the scenes: How Waltz got the boot

How it happened: In Trump's mind, Signalgate was the first time he was unable to control the narrative or win the day, top advisers tell Axios.

• Waltz came to symbolize Trump's first obstacle. Momentum is so important to the president that he emphasized the concept in his advice last night to University of Alabama graduates.
• Waltz also hadn't collaborated well with others in the White House. (Go deeper.)

It was Waltz who added Jeffrey Goldberg, editor-in-chief of The Atlantic, to a thread in the messaging app Signal where attack plans were discussed. The embarrassing debacle embittered Trump.

• "He just came to look at Waltz like he was bad luck, bad news," a White House official familiar with Trump's thinking told Axios. "There was nothing Mike could do at that point."

As criticism rose about tariffs, Trump would occasionally return to fuming about Waltz — who had nothing to do with that policy.

• Immediately after the Signal story broke, Trump wanted to fire Waltz but held off, Axios reported at the time.

Behind the scenes: Trump was ready to fire Waltz or force him out. White House chief of staff Susie Wiles advocated for the soft landing of ambassador to the UN — an open role after Trump pulled the nomination of Rep. Elise Stefanik (R-N.Y.).

• Exactly what triggered Trump's first big personnel shakeup of the term was unknown even to some close aides. But Trump makes decisions by his gut.

What's next: Waltz now will have to undergo Senate confirmation for UN ambassador — meaning a raw reliving of questions about the national security team's use of Signal.