By Mark Landler and Helene Cooper, New York Times

WASHINGTON — Donald Trump ran for president boasting that he knew more about fighting terrorists than America’s generals.

But now that Trump is the president-elect, he is spending a great deal of his time with retired generals, and those of a particular breed: commanders who, when they served, were often at odds with President Barack Obama.

One has been named as Trump’s national security adviser, and several others are candidates for coveted positions in his Cabinet or are advising him on how to confront the world’s greatest threats. They would give his foreign policy a far more aggressive cast than Obama’s.

Turning to the retired officers reflects Trump’s preference for having strong, even swaggering, men around him. But it worries national security experts and even other retired generals, who say that if Trump stacks critical jobs purely with warriors, it could lead to an undue emphasis on military force in U.S. foreign policy.

“If you have too many generals in the kitchen, the dish is likely to be baked with even more military instruments inside,” said John A. Nagl, a retired Army lieutenant colonel and an expert in counterinsurgency strategy. “I’m not sure that’s the recipe the United States needs for every foreign policy meal.”

Trump’s inclination toward generals in top jobs also runs counter to the credo of civilian control of the military — a constitutionally enshrined principle that some say safeguards the U.S. from becoming another Pakistan or Turkey, where the military is a political player.

In the last week, Trump has met with James N. Mattis, a retired Marine Corps general, who has emerged as a leading candidate for secretary of defense; John Kelly, another retired Marine general, who is in the running for secretary of state; Jack Keane, a former Army vice chief of staff, who said he had turned down an offer from Vice President-elect Mike Pence to run the Pentagon; and Adm. Michael S. Rogers, the director of the National Security Agency, who, of those under consideration, is the only job candidate still on active duty and is a prospect for director of national intelligence.

Trump’s closest foreign policy adviser is Lt. Gen. Michael T. Flynn, a former head of the Defense Intelligence Agency, who was named as national security adviser on Friday. Trump’s aides also sounded out Gen. Stanley A. McChrystal, a former commander in Afghanistan, for defense secretary, according to people briefed on the transition process, and they briefly mulled David H. Petraeus, a former director of the CIA who also served as commander in Afghanistan, for secretary of state.

What several of these generals share is a rocky history in the Obama administration. The White House forced Flynn out of his last job, citing poor management of an agency with 20,000 employees. Mattis, who oversaw military operations in the Middle East and Southwest Asia from 2010 to 2013, had his tour cut short by the Obama administration, which believed he was too hawkish on Iran. Kelly opposed the administration’s plan to shut down the military prison at Guantánamo Bay, Cuba.

Obama fired McChrystal after he disparaged other officials in an interview with Rolling Stone, while Petraeus, as CIA director, failed in his effort to persuade Obama to supply arms to moderate rebels in Syria. Obama eventually did provide the weapons, and Petraeus’ resignation in 2012 was prompted by an unrelated scandal involving his disclosure of classified material.

For Trump, the strained history of these generals with his predecessor is less a liability than a credential.

“He is sticking his tongue out at the prior administration,” said Richard H. Kohn, a professor emeritus of history at the University of North Carolina at Chapel Hill who has written about civilian control of the military. “He is inherently distrustful of the military people that the system throws up at him. He thinks they’re not strong people. They are Obama’s generals.”

Trump might also view generals in his administration as a political asset. At a time when trust in institutions like Congress and the news media is eroding, the military remains enduringly popular. Mattis, in particular, commands wide respect from uniformed men and women — especially Marines, who revere him as one of their own. On Sunday, Trump wrote on Twitter that Mattis was a “true General’s General.”

Stephen Bannon, the president-elect’s chief strategist, said the incoming administration was looking at potential Cabinet officials with combat experience so that people who had fought in wars would be making decisions about whether to commit the country to more of them.

“The generation that has fought in these wars is now coming into leadership,” said Bannon, a Navy officer earlier in his career. “These people, all patriots, come with a shared experience that is incredibly important.”

But Mattis’ appointment — because he would oversee the department where he used to serve — would raise legal and political questions. He would need a congressional waiver because the law requires a seven-year waiting period between active duty and serving as defense secretary, and he retired from the Marines only in 2013.

The last time a waiver was used was in 1950, when Gen. George C. Marshall, then five years out of service, received one to be defense secretary for President Harry S. Truman.

There is little doubt that Mattis, who is well liked by Sen. John McCain, R-Ariz., and the chairman of the Senate Armed Services Committee, will be able to obtain that waiver. McCain appeared to try to hasten the process, issuing a statement Monday in which he said he was “pleased that the president-elect found General Jim Mattis as impressive as I have.”

Some experts said that installing a general at the top of the Pentagon could muddy the principle of civilian control of the military.

“The president and the secretary of defense are the two leading figures in the chain of command,” said Peter D. Feaver, a political science professor at Duke University who served in the George W. Bush administration. “When they are civilians, that embodies that principle.”

Gen. Carter F. Ham, the retired head of the U.S. military’s Africa command, said the question of whether Mattis was qualified to lead the Pentagon was a “slam dunk — he absolutely is.” But the reason the waiver “was put into law is that we are not a militaristic society, nor do we want to be,” he said. “The idea of senior military officers assuming senior positions in the civilian government — that is worthy of debate.”

Beyond the constitutional issues, there are bureaucratic hurdles that could stymie a military officer. The defense secretary must navigate the politics of the White House and Congress while balancing — and in some cases, resisting — the views of his former colleagues.

In a famous example, Defense Secretary Robert M. Gates overruled his many senior generals, forcing through the development and purchase of more maneuverable and heavily armored mine-resistant, ambush-protected vehicles, known as MRAPs, to stem a rising tide of casualties from roadside bombs in Afghanistan and Iraq.

People close to Trump said they were sensitive to the dangers of having too many military officers. They noted that there were other candidates for defense secretary, including Jim Talent, a former Republican senator from Missouri. The goal, they said, was to end up with a mix of people.

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