WASHINGTON — President-elect Donald J. Trump’s selection of Gen. James N. Mattis as defense secretary signals a more assertive American posture in the Middle East — one that people close to him say would most likely include more American troops on the ground in Iraq and Afghanistan, more Navy patrols in the Persian Gulf and more fighter jets in the Middle East.

“The closest thing we have to Gen. George Patton,” Mr. Trump said in announcing his selection on Thursday night. At first glance, the similarity is there: Like General Mattis, General Patton, who led American troops into Nazi Germany during World War II, was a colorful, hard-charging advocate of aggressive offensive action.

But officials who know General Mattis caution that he views a tough American posture overseas as something to deter war with potential foes like Iran, not to start one. And although he was so hawkish on Iran as head of United States Central Command from 2010 to 2013 that the Obama administration cut short his tour, General Mattis has since said that tearing up the Iran nuclear agreement, as Mr. Trump has vowed to do, would hurt the United States.

General Mattis now favors working closely with allies to strictly enforce the deal.

“I don’t think that we can take advantage of some new president, Republican or Democrat, and say we’re not going to live up to our word on this agreement,” General Mattis, a retired Marine, said in April. “I believe we would be alone if we did, and unilateral economic sanctions from us would not have anywhere near the impact of an allied approach to this.”

Military officers and foreign policy specialists say General Mattis will most likely advocate some buildup in the 6,000 American troops currently in Iraq but will argue for the increase only if it is tied to an overall strategy for the country after American-backed Iraqi security forces defeat Islamic State militants in Mosul, as expected.

“He does not fundamentally simplify the world in hard-power military terms,” said Michael E. O’Hanlon, a senior fellow in foreign policy at the Brookings Institution. And unlike other candidates whom Mr. Trump has considered for top national security posts in his administration, like Mitt Romney or Rudolph W. Giuliani, General Mattis “will have the credibility to tell Trump when a military solution will not work,” Mr. O’Hanlon said.

To that end, General Mattis has advocated working more closely with allies in the region to strengthen ties with their spy agencies and to expand naval exercises, including international efforts to stop Iran from using mines to cripple the flow of oil and other global traffic.

As the head of Central Command, the general pushed for “maintaining and diversifying the U.S. military presence in the Middle East amidst the Iraq drawdown, budget pressures and the rebalance to Asia,” said Derek Chollet, a former assistant secretary of defense in the Obama administration.

Three years later, General Mattis’s hard-line views on Iran have not softened as he points out the country’s “malign influence,” whether it is shipping weapons to rebels in Yemen or training Shiite militias bound for Syria or Iraq.

“The Iranian regime, in my mind, is the single most enduring threat to stability and peace in the Middle East,” General Mattis said in April at the Center for Strategic and International Studies, a Washington think tank. “Iran is not a nation-state; it’s a revolutionary cause devoted to mayhem.”

Sunni allies in the Persian Gulf, who have criticized the Obama administration for improving relations with Shiite-majority Iran, cheered General Mattis’s selection.

“He brings a fingertip feel and familiarity to the region that will serve our interests much better, but it doesn’t necessarily mean we’ll pour in a whole bunch more troops,” said Vice Adm. Mark I. Fox, who served under General Mattis as head of the Navy’s Fifth Fleet in the Persian Gulf.

Normally, an examination of a president-elect’s top pick for the Pentagon would look at the policy differences between the incoming president and the nominee. But it is difficult in this case because Mr. Trump has publicly expressed so many conflicting views on national security. (He supported the 2003 American-led invasion of Iraq but says he did not. He said during the campaign that he would reinstate waterboarding “in a heartbeat” but appeared to backtrack on that assertion last week during an interview with The New York Times, after listening, he said, to General Mattis’s views.)

Still, it is difficult to see how the two men would find agreement on Mr. Trump’s professed admiration for President Vladimir V. Putin of Russia, given that General Mattis, like most military officers at the Pentagon, has a healthy skepticism for the Russian leader.

General Mattis’s selection also raises potentially consequential questions about the issue of civilian control of the military, a standard set by the framers of the Constitution. The founding fathers were adamant that the country be led by a civilian president who exercises control over the military as commander in chief so that the nation will not become a military state. Similarly, the leader of the Defense Department is always a civilian who is superior to the chairman of the Joint Chiefs of Staff, the nation’s top military official.

General Mattis’s retirement from the Marines in 2013 means that he would need a congressional waiver to be defense secretary because American law requires a seven-year waiting period between active duty and serving in that role. There is little doubt that General Mattis, who is well liked by Senator John McCain, Republican of Arizona and the head of the Armed Services Committee, will be able to get that waiver.

Mr. McCain said he was pleased with the general’s nomination, saying in a statement that he “looked forward to moving forward with the confirmation process as soon as possible in the new Congress.”

But Senator Kirsten E. Gillibrand, Democrat of New York, said she plans to vote against the waiver. “While I deeply respect General Mattis’s service,” she said in a statement Thursday night, “civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule.”

The congressional waiver has not been used in 66 years, since Gen. George C. Marshall, then five years out of active service as the Army chief of staff, received a waiver to be President Harry S. Truman’s defense secretary.

General Mattis’s selection has also resurfaced an accusation from “The Only Thing Worth Dying For,” a 2010 book by Eric Blehm, who wrote about a group of Green Berets after they were hit by an American smart bomb in Afghanistan in 2001. In the book, Mr. Blehm quotes a former Army Special Forces officer accusing General Mattis, then a brigadier general, of refusing to send helicopters to rescue the Green Berets. The general declined to be interviewed for the book.

Mr. Trump has so far shown an affinity for retired military officers as he assembles his government. Lt. Gen. Michael T. Flynn, a former head of the Defense Intelligence Agency, has been named national security adviser, while Mr. Trump’s aides sounded out Gen. Stanley A. McChrystal, a former commander in Afghanistan, for defense secretary. David H. Petraeus, a former C.I.A. director and a retired four-star Army officer who served as the military’s top commander in Afghanistan and Iraq, has been under consideration for secretary of state.

But the nomination of General Mattis could make Mr. Petraeus’s selection less likely, because Democrats will object to the prevalence of retired generals in jobs that normally go to civilians.

While General Mattis is revered by Marines and enlisted soldiers, several military officials interviewed on Friday raised questions about whether his proven combat skills as a military leader will be the right tools to overcome the calcified bureaucracy that is the Defense Department.

A defense secretary must navigate the politics of the White House and Congress while balancing and, in some cases, steering the views of his top military officers.

When Robert M. Gates served as defense secretary during the most bloody periods of the wars in Iraq and Afghanistan, he overruled senior generals and forced through a speedy development of more heavily armored MRAPs — mine-resistant, ambush-protected vehicles — to save American lives from roadside bombs. Generals outside the war zone were reluctant to spend so much money on a new vehicle that had not been part of their long-term planning and budgeting.


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