WASHINGTON — Six days before President Trump chose Robert O’Brien as his national security advisor in September, the president said the job would be simple.

“You know why it’s easy?” Trump told reporters. “Because I make all the decisions. They don’t have to work.”

As a key White House advisor, O’Brien clearly works — but he meets Trump’s other job requirements: He avoids publicity, he gets out of the way on policy decisions, and he dismisses employees Trump views as meddlesome.

Unlike his predecessors, John Bolton or H.R. McMaster, who pursued their own agendas or tried to block some of Trump’s impulses, O’Brien has taken a wrecking ball to parts of the National Security Council, the intelligence and foreign policy hub of the White House, to satisfy a president who doesn’t trust experts, is suspicious of career government employees, and acts on his own whims.

Long before O’Brien had security guards escort Lt. Col. Alexander Vindman, a key witness in Trump’s impeachment case, and his twin brother, Lt. Col. Yevgeny Vindman, an ethics lawyer, out of the White House on Friday, O’Brien had dismissed or transferred about 70 people, or about one-third of those employed by or temporarily assigned to the NSC, according to senior administration officials.

O’Brien told a Washington think tank on Tuesday that his efforts to trim the staff would conclude this week, and aides said the final cuts would involve only a few more employees. O’Brien denied that his downsizing of the NSC was an effort to dismantle what Trump has called the “deep state.”

O’Brien said his primary aim isn’t to remove career government employees and other professionals in favor of Trump loyalists. But he conceded that the realignment has increased the proportion of politically appointed staffers.

“The president is entitled to a staff that he has confidence in and that he believes will execute his policies,” O’Brien, a former Los Angeles lawyer, said to the Atlantic Council, a nonpartisan foreign policy think tank in Washington.

Former NSC employees view O’Brien’s cuts as damaging the White House ability to formulate, vet and coordinate U.S. policy on a broad array of fronts — while Trump often sees the professional staff as a roadblock.

Trump “has a high degree of paranoia that he has a bloated National Security Council full of deep-state minders who are there to undermine him, not to fulfill his national security policy,” said a former NSC official who was a political appointee under Trump, but was not among those removed involuntarily and spoke on condition of anonymity to discuss internal matters.

“It sort of reflects a Trumpified version of government, which is you don’t really need a lot of staff,” said John Gans, author of “White House Warriors: How the National Security Council has Transformed the American Way of War.”

“It’s not about getting everybody in the room,” he added. Trump, he said, prefers to say that “we’ll make the decision and everybody else can catch up.”

Foreign policy veterans warn that the size and speed of O’Brien’s cuts have left a vital part of the White House ill-equipped to respond to crises like the spreading coronavirus, which has killed more than 1,000 people in China.

Others faulted the NSC for lacking a coordinated response to Trump’s shifting calls for a U.S. troop withdrawal from Syria, for its rocky efforts to limit Chinese tech giant Huawei’s growing role in global telecommunications infrastructure, and for the U.S. drone strike that killed Iranian Quds Force commander Qasem Suleimani in Iraq, nearly sparking a war with Iran.

Trump’s disdain for traditional process was on full display during the House impeachment inquiry as career professionals from the NSC, the State Department and elsewhere described a shadow effort led by Trump’s private attorney, Rudolph W. Giuliani, to prioritize Trump’s personal political goals over official U.S. policy in Ukraine.

Experts who keep close tabs on the NSC say Trump relies on the country and subject experts less than other presidents have done.

“You could have the best policy-making process in the world and you still have a president who acts on impulse and flies off the handle,” said Daniel Drezner, editor of “Avoiding Trivia: The Role of Strategic Planning in American Foreign Policy.”

“Some of this flows directly from the president wanting to do what he’s told he can’t do,” said Peter Feaver, who served on NSC staffs for Presidents Clinton and George W. Bush.

The staffing cuts have strengthened some of Trump’s most loyal Cabinet officials, especially Secretary of State Michael R. Pompeo, who has played a greater role in defense decisions than his predecessors, and to a lesser extent Treasury Secretary Steven T. Mnuchin, who leads Trump’s efforts to use economic sanctions and trade deals to pressure other nations , according to former officials and others who study the NSC.

O’Brien took the job as national security advisor with a promise to reduce the NSC, offering himself to Trump as the antithesis of his immediate predecessor, Bolton, a veteran brawler with strong policy views who clashed with the president on Syria, North Korea and Iran, and left on acrimonious terms.

O’Brien’s promise to shrink his own fiefdom endeared him to Trump, and to Pompeo, who was O’Brien’s boss at the State Department, where he had worked as a hostage negotiator.

The NSC was formed after World War II to synthesize and coordinate the foreign policy and intelligence agencies that report to the White House. The national security advisor is supposed to serve as an honest broker among the factionalized bureaucracies.

As threats grew more complex and Congress and the public demanded a faster White House response to crises, the NSC steadily grew, peaking at more than 200 people under President Obama.

Most of the staff are analysts on loan from the State Department and the Pentagon, but the NSC also includes specialists from the CIA and other intelligence agencies, and the departments of Homeland Security, Justice and Treasury.

The career officials are usually led by the president’s political appointees, often creating a built-in friction and frequent turf wars.

Trump’s first national security advisor, Michael Flynn, flamed out quickly and later pleaded guilty to lying to the FBI about his contacts with Russians. He is awaiting sentencing.

McMaster, who replaced Flynn, was a retired Army lieutenant general with a chest full of combat medals and other awards. He tried to run a traditional operation, holding large meetings and giving Trump lengthy policy presentations that the president grew to resent.

Bolton, who replaced McMaster, was a Fox News pundit and Republican archconservative dating back to the Reagan administration.

He was more proactive than McMaster, and sought to block or detour some of Trump’s initiatives. He was convinced that Trump’s nuclear talks with North Korean leader Kim Jong Un would fail, and opposed Trump’s efforts to pull U.S. troops out of Syria.

Bolton favored smaller meetings so he could control the process, making as many policy decisions as he could “so they could run a relatively normal government with an abnormal president,” said Colin H. Kahl, who served on Obama’s NSC.

O’Brien spent much of his adult life as an attorney in private practice, serving as California managing partner of Arent Fox LLP for seven years, and is not known for strong views on foreign policy or national security issues.

He advised two candidates who had more establishment backing in the 2016 Republican primary — then-Wisconsin Gov. Scott Walker and Sen. Ted Cruz of Texas — but he later impressed Trump by leading the State Department team that secured the release of several Americans detained abroad, including evangelical pastor Andrew Brunson from Turkey in 2018.

Unlike Bolton, who has a sarcastic edge and a bushy mustache, O’Brien speaks in understated tones and wears a neat pocket square to match his tie. By most accounts, Trump, who often judges people by how they look on television, finds him easy to be around.

But like all of Trump’s advisors, O’Brien is forced to adjust his public comments to match the president’s ever-changing claims and declarations.

That was apparent this week after Trump suggested Alexander Vindman, a highly decorated active-duty Army officer who was the top Ukraine expert at the NSC before he was ordered off the White House grounds last week, should be punished by the military for testifying under subpoena in the House impeachment inquiry.

“I obviously was unhappy with the job he did,” Trump told reporters Tuesday.

“That’s going to be up to the military, we’ll have to see, but if you look at what happened, they’re going to certainly, I would imagine, take a look at that,” he added.

The Army has reassigned Vindman to the Army War College, and his brother to the judge advocate’s office. Their lawyer, David Pressman, has angrily denied that either acted inappropriately, saying they followed the chain of command and were victimized for telling the truth.

In his comments at the Atlantic Council, however, O’Brien said that he had ordered the Vindman brothers out of the White House, saying their services “were no longer needed” and suggesting they had tried to usurp the president’s authority.

“We’re not some banana republic where lieutenant colonels get together and decide what the policy is or should be,” he said.