Of all the members of President Obama’s new national security team, Lt. Gen. John R. Allen, tapped as the fourth commander in two years to run the war in Afghanistan, may have the toughest assignment.

In the coming days, Obama is expected to make a key strategic decision about how many troops the United States can afford to withdraw from Afghanistan without giving ground to the Taliban. To execute that high-stakes plan, he’ll turn to Allen, 57, a low-profile Marine with no combat experience in Afghanistan but with a reputation as a military turnaround specialist.

Unlike the household name he is slotted to replace, Army Gen. David H. Petraeus, Allen has served in relative public obscurity for the past three years, as deputy commander of U.S. Central Command in Tampa.

Like Petraeus, however, Allen is known in defense circles as a scholarly leader whose strategic acumen helped reverse the tide of the war in Iraq. While serving as deputy commander in Anbar province in 2007 and 2008, Allen orchestrated the Sunni Awakening, the long-odds campaign to persuade hostile tribes to side with the U.S. military against al-Qaeda in Iraq and foreign fighters who were fueling the insurgency.

Obama will need him to pull off a similar feat, on a greater scale, with Afghanistan’s notoriously fractious tribes and warlords as international pressure builds to find a quick political solution to the decade-long war.

Even some of Allen’s most fervent supporters wonder whether he will find himself in a no-win position come September, when he is scheduled to take over from Petraeus as commander of 140,000 U.S. and NATO troops. The Senate is expected to hold confirmation hearings on his appointment and his promotion to four-star general by next month.

Confronting a resilient enemy on the battlefield may be the least of his problems. Allen will also have to contend with Afghanistan’s mercurial president, Hamid Karzai; an uncertain partner in Pakistan; and lawmakers in Congress who are itching to bring home U.S. troops.

“Will John be able to do in Afghanistan what he did in Iraq? It depends on the political landscape,” said James L. Williams, a retired Marine major general who has known Allen since they were young officers. “The question is, will he have the supporting cast in Washington, D.C., that he needs, as well as with our European allies?”

“I think that’s questionable,” Williams said.

For now, Allen has the support of his commander in chief, Obama, who announced Allen’s promotion April 28 as part of a broader turnover of his national security team, including the nominations of Leon Panetta as defense secretary, Petraeus as CIA director and Ryan C. Crocker as ambassador to Afghanistan.

Obama, however, has run through a string of generals in Afghanistan since taking office 21 / 2 years ago.

Gen. David D. McKiernan was fired in May 2009 after Defense Secretary Robert M. Gates said someone with “fresh eyes” was necessary. Gen. Stanley A. McChrystal was forced to resign in June 2010 after he and his staff members were quoted as making disparaging remarks about civilian leaders. Petraeus maintained favor with the White House, but he was not intended as a long-term replacement and is leaving after 14 months.

Allen declined an interview request for this article, citing his pending nomination in the Senate.

First Marine in top job

If he is confirmed, he will be the first Marine to command all U.S. forces in either Afghanistan or Iraq.

He was selected over Army Lt. Gen. David M. Rodriguez, a longtime Afghanistan hand who has served as deputy commander under McChrystal and Petraeus. While Rodriguez is seen as an effective combat leader and as highly knowledgeable about Afghanistan’s complex political terrain, senior Pentagon officials questioned whether he had the communication skills to maintain support for the war on the home front.

Allen, by contrast, has not served any combat tours in Afghanistan, although he is familiar with the country and the war because of his three years as deputy leader of Central Command, which oversees military operations in the Middle East and part of South Asia. He will rely on a new deputy commander, Army Lt. Gen. Curtis M. Scaparrotti, to run the day-to-day war operations.

Perhaps most important, Allen’s backers said, he has built a close relationship with Obama, having regularly briefed the president while at Central Command. They said Allen was a natural choice to take over from Petraeus, under whom he served in Tampa as well as in Iraq.

“His reputation is pretty well known in the White House,” said Gen. Joseph F. Dunford Jr., the assistant commandant of the Marine Corps. “You’re fair to say he’s not well known [to the public], but it’s unfair to say he doesn’t know the fight.”

Working in the limelight will be a change for Allen. During his stints as commander in Iraq and Afghanistan, Petraeus relentlessly cultivated ties back in Washington with lawmakers, think tank specialists and journalists. McChrystal tried to do the same but crashed after he and his staff members made careless comments to a Rolling Stone reporter.

Charles C. Krulak, a retired Marine commandant and a mentor to Allen, predicted that his protege will adapt quickly and has probably already developed a strategy for tackling politics and public relations.

“He is not one to toot his own horn, which is probably why many people don’t know much about him,” Krulak said. “But I think he’s going to knock your socks off, and here’s why: He’s about as transparent an individual as you will find. He’ll give you the good, the bad and the ugly, no matter the consequences.”

A long line of service

Allen comes from a family with an extensive history of American military service dating to colonial times. His grandfather, a combat engineer, fought on the Western Front during World War I and was badly gassed. His father, Joseph K. Allen, was a crewman on the USS Kearny when it was torpedoed by a German submarine two months before the United States entered World War II.

His father later joined the invasion of Italy, was present in Tokyo Bay when Japan surrendered, served during the Korean War and retired as a lieutenant commander.

After graduating from the Naval Academy in 1976 and joining the Marine Corps as an infantry officer, Allen earned a master’s degree from Georgetown University. He became the first Marine officer to be inducted as a term member of the Council on Foreign Relations and the first Marine to return to the Naval Academy to serve as commandant of midshipmen, a job equivalent to dean of students.

In a letter to Naval Academy students when he was a brigadier general, Allen said he had been profoundly affected by reading classic military historians, including John Keegan, Barbara Tuchman, Stephen Ambrose and Russell Weigley. “If my house were on fire and we were all running for our lives, I would first save my family, and then all my volumes by these writers,” he wrote.

“He can quote from ‘Henry V’ without sounding bombastic,” said Nathaniel C. Fick, chief executive of the Center for a New American Security and a former Marine officer who trained under Allen.

While serving as deputy commander in Anbar province, Allen told Thomas E. Ricks, author of “The Gamble” and a former Washington Post military reporter, that if he hadn’t been a Marine, he would have liked to have been an archaeologist. And not just any archaeologist, but one along the lines of Gertrude Bell, the British adventurer who helped draw the borders of present-day Iraq in the 1920s.

“She had the life I perhaps would have liked to have had,” Allen said.