WASHINGTON >> President Donald Trump on Thursday removed his national security adviser, Michael Waltz, and nominated him as ambassador to the United Nations, the first significant personnel overhaul of top White House aides and the kind of shake-up that Trump had sought to avoid in his second term.

Secretary of State Marco Rubio will serve as interim national security adviser and will remain the nation’s top diplomat, Trump said.

“From his time in uniform on the battlefield, in Congress and, as my National Security Advisor, Mike Waltz has worked hard to put our Nation’s Interests first,” Trump wrote in a post on social media. “I know he will do the same in his new role.”

Signal app flap

Waltz had been on thin ice as national security adviser for months, but his position became more precarious after revelations that he organized a group chat on the commercial messaging app Signal to discuss a sensitive military operation in Yemen and accidentally included a journalist in the conversation. A news article on the chat, and the chat’s contents, appeared in the following days.Waltz’s job was saved at the time in part because Trump, aides said, did not want to be seen as giving in to the news media by firing Waltz.

By then, most of Trump’s advisers had already viewed him as too hawkish to work for a president who campaigned as a skeptic of American intervention abroad and was eager to reach a nuclear deal with Iran and normalize relations with Russia.

Waltz’s deputy, Alex Wong, who worked on North Korea issues in Trump’s first term and who is considered a more moderate Republican with substantial national security experience, is also expected to be removed, according to a senior administration official with knowledge of the situation. The official and others spoke on the condition of anonymity to describe the internal discussions.

Rubio’s role

Rubio now has a lengthy list of job titles. He holds the position of secretary of state and national security adviser, something that no other official has done simultaneously since Henry Kissinger held both titles under the Nixon and Ford administrations in the 1970s. Rubio has also been serving as the acting head of both the gutted U.S. Agency for International Development and the National Archives.

The Kissinger experiment has not been considered a success by most historians. The national security adviser is supposed to help adjudicate among competing arguments inside a national security establishment, and thus must often resolve differences among the State Department, the Pentagon and the intelligence agencies, among others. Kissinger was ultimately removed from the post of national security adviser and replaced with Brent Scowcroft.

But Trump is not running a second administration that has much of a true historical parallel.

One person with knowledge of the discussions said Rubio had indicated some time ago that he would be willing to serve for roughly six months if he were asked to do so.

Waltz did not join Trump for a trip to Michigan marking his first 100 days in office as had been planned, although the person with knowledge of the events said that was because he was handling a foreign policy matter.

The decision to remove Waltz was so abrupt that many on Trump’s staff only learned of it Thursday morning, as it was being discussed among senior officials in the White House. By midmorning, top aides were told it was true that he would be removed, and to await Trump’s statement, according to the person with knowledge of the events.

Tammy Bruce, the State Department spokesperson, learned of Rubio’s new role during a briefing with reporters, after Trump posted about it.

What’s next

The selection of the next national security adviser will be a critical one, at a moment when the president’s top national security and foreign policy aides have differed sharply on how to handle three of America’s most potent adversaries: China, Russia and Iran.

Trump is making the change just two weeks before his first major trip abroad, to Saudi Arabia and other Arab capitals in the Middle East, and in the midst of tense negotiations with Moscow and Tehran. Waltz joined him in Rome a week ago, for the funeral of Pope Francis, and was in the background of the meeting in St. Peter’s Basilica with President Volodymyr Zelenskyy of Ukraine.

Waltz, a veteran who had favored a muscular U.S. foreign policy for many years before Trump’s rise, had struggled to prove his bona fides to his base.

He also barely had time to organize a staff, much less tackle the hardest issues facing the president. Ordinarily a new national security adviser writes the administration’s national security strategy, often a yearlong process that involves hashing out differences among government agencies and Cabinet members.

On some major issues — including how to engage China on its ambitions in the Indo-Pacific, its fast-growing nuclear program, its sophisticated cyberattacks on the United States and its allies and its effort to reunite Taiwan with the mainland — the administration has barely gotten started.

But Waltz was also seen by the senior team closest to Trump as struggling to show he could manage the role while managing his relationship with the president. One senior official said he had at various points been seen as seeking to minimize Susie Wiles, the White House chief of staff and the adviser in the building who is closest to Trump.

On Thursday morning, just as word of his ouster was beginning to circulate in national security circles in Washington, Waltz appeared on Fox News. A person with direct knowledge of the situation said that Waltz was aware that something was going to happen before he went on.

Waltz, who never made the public evolution toward Trump’s foreign policy views that Rubio did over the past several years, has been arguing internally for sharp sanctions against Russia if it fails to agree to a ceasefire with Ukraine. Waltz made that suggestion as an option for handling Russia as recently as Monday at a meeting with the president and senior members of his national security team, according to a person with knowledge of what took place.

So far, Trump, who has repeatedly voiced skepticism of the long-running United States efforts helping to defend Ukraine, has been reluctant to take anything but symbolic action against Russia, though at times he has threatened on social media to impose sanctions and tariffs.