After Michael Waltz, the national security adviser, inadvertently included The Atlantic’s editor in chief in a group chat about military attack plans on the Signal messaging app, he found himself on very thin ice with his boss.

But President Donald Trump and his advisers were loath to take a political hit by firing Waltz, especially within the first 100 days of the new administration. The 100-day mark passed yesterday. Today, the administration’s 101st day, Trump acted against his national security adviser, removing Waltz along with his principal deputy, Alex Wong.

Hours after the news of Waltz’s removal broke, Trump wrote on Truth Social that he would nominate the former Florida congressman as ambassador to the United Nations. Trump said he would give Marco Rubio, his secretary of state, the added responsibilities of the national security adviser—at least on an interim basis. The dual roles were last held by Henry Kissinger from 1973 to 1975.

Waltz is the first top aide to be replaced in Trump’s second term. The overhaul echoes the dismissal of Michael Flynn, Trump’s first national security adviser, who was fired in February 2017 for lying to Vice President Mike Pence about discussions he held with the Russian ambassador. Trump ultimately had four national security advisers in his first term.

The origins of Waltz’s offenses, according to people familiar with Trump’s thinking, predated the Signal chat. He didn’t work well with other senior members of Trump’s team, they said, and couldn’t prove to the president that he was able to manage his own staff. This account of Trump’s decision to shake up his national-security team is based on interviews with 14 current and former White House officials and outside advisers, all of whom spoke on the condition of anonymity to discuss sensitive internal deliberations.

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In some ways, the officials said, Waltz was never a good fit for Trump.

His first stint in the executive branch was as an aide to Vice President Dick Cheney, whose hawkish foreign-policy views have fallen out of favor in Trump’s Republican Party. During Trump’s first campaign for president, in 2016, Waltz appeared in an anti-Trump ad to accuse him of dodging the Vietnam draft and exhort fellow conservatives to “stop Trump now.” His first reaction to Russia’s invasion of Ukraine in 2022 was to accuse President Vladimir Putin of “despicable war crimes,” the kind of unambiguous criticism of Moscow that Trump has refused to utter.

A former Green Beret, Waltz had, for a time during last year’s campaign, endeared himself to Trump by assiduously defending him on Fox News. During the transition, Waltz was frequently spotted at Mar-a-Lago, though rarely seated at Trump’s table. Despite shifting his views to align with “America First” dogma, however, Waltz never found his way into the president’s inner circle, and was never trusted as a loyal foot soldier.

He clashed with Trump’s chief of staff, Susie Wiles, who found him dismissive, people familiar with the dynamics told us. And despite Waltz’s efforts to banish career officials whose service at the National Security Council began under Joe Biden, his staff remained a target for the powerful White House personnel office, which viewed the NSC as fertile ground for rooting out officials not fully committed to Trump’s agenda. On substantive foreign-policy issues, too, distance remained between Waltz and other influential voices in Trump’s inner circle. He was one of the few advisers consistently pushing for escalating sanctions against Russia if Moscow didn’t cooperate in peace talks.

The personnel overhaul followed months of chaos at the National Security Council, a highly sensitive part of the U.S. government that provides a forum for the president to consider pressing national-security and foreign-policy issues with senior advisers and the Cabinet. The instability began almost instantly, when Waltz’s team moved in the first week of the new administration to dismiss scores of career officials detailed to the NSC—a priority for Trump, who believes that NSC staff thwarted his agenda in his first term. The dismissals hindered core functions of the council, as whole offices sat vacant. Meanwhile, new hiring was delayed by the White House personnel office, which is typically uninvolved in internal NSC hiring.

The dysfunction burst into public view in March and April and proceeded to undermine Waltz’s grip on his staff.

In March, Waltz accidentally added The Atlantic’s editor in chief, Jeffrey Goldberg, to a group chat on Signal about a forthcoming military attack on Houthi militants in Yemen. Waltz struggled to explain the blunder, at one point describing why he had Goldberg’s number saved in his phone by saying, “It gets sucked in.” The problems for Waltz began in earnest after the Signal controversy, one former NSC official told us. “There wasn’t a sense of a cloud of suspicion hanging over him,” the former official said. “It was Signalgate that made him vulnerable.”

In April, Trump ordered the dismissal of numerous NSC officials based on the advice of Laura Loomer, the far-right activist who rose to prominence by making incendiary anti-Muslim claims and who last year shared a video that labeled 9/11 an “inside job.” In an Oval Office meeting with Trump, Loomer accused senior members of Waltz’s staff of disloyalty. Waltz, who has an office in the West Wing, wasn’t even present for the beginning of the meeting. When he joined, the national security adviser protested that he had carefully vetted the members of his team.

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The spectacle, current and former officials told us, made clear that Waltz had lost control of his own staff. Waltz was originally slated to attend Trump’s Michigan rally this week to mark his first 100 days but was ultimately directed not to go.

“He was hired primarily to look good on TV while defending the president’s decisions,” an official from Trump’s first term, who remains in contact with the White House, told us. “He failed at that; he was a bad messenger, and, off TV, he never was seen as being bought in.”

The national security adviser’s dismissal elevated the anxiety of key U.S. allies, who saw him as a stabilizing force in the administration because of his pro-NATO views and the support for Ukraine he had voiced as a member of Congress. Western officials were already alarmed by the dismissals of NSC staff following Loomer’s appearance in the Oval Office, as well as by the White House’s move to block a retired CIA officer for a key position at the agency because he was deemed too supportive of Ukraine.

Some officials from allied nations told us recently they were concerned that loyalty tests were driving personnel decisions, particularly at lower levels of the national-security apparatus, which are normally staffed by career personnel and are not subject to such overt political influence.

If Waltz sensed this week that his time in the White House was coming to an end, he didn’t let on. In an opinion piece published online Tuesday, he outlined “100 Days of National Security Wins.” And on the morning of his dismissal, he presented himself as a happy warrior in an appearance on Fox & Friends, hailing enhanced military recruitment and saying of Trump, who would soon replace him, “This is leadership at its finest.”

Michael Scherer contributed to this report.