And now the sex scandal that draws them together — Petraeus’ career toppled and Marine Gen. John Allen’s possibly on hold — also has placed greater attention to the quick turnover of American battlefield chiefs in the 11-year war.

Nearly two dozen generals have commanded troops from the United States and the NATO-led International Security Assistance Force, or ISAF, since the American invasion in late 2001 — with five U.S. generals running both commands in the past five years alone.

There is no firm evidence the Pentagon’s revolving door in Afghanistan has posed any significant obstacles for U.S. troops, but some military analysts suggest the frequent changes at the top create potential breaks in continuity in the critical cooperation with the Afghan political leadership and security officials.

“The learning curve is pretty steep,” said retired Lt. Gen. David Barno, who commanded U.S. forces in Afghanistan in 2003 and 2004. “One of the critical coins of the realm in being effective in this kind of environment is relationships among your allies, relationships with the host nation, and with the Afghans.”

Iraq also had regular command changes, including Petraeus in charge during the U.S. troop “surge” in 2007 that helped dislodge insurgent control from key areas. But the war strategy in Afghanistan has, at many times, been even more complex as fronts shift and Taliban fighters regain strength.

“Rotating top commanders on an annual basis makes no management sense,” Thomas E. Ricks, a senior fellow at the Center for a New American Security, wrote in an opinion piece published recently in The New York Times.

“Imagine trying to run a corporation by swapping the senior executives every year,” he continued. “Or imagine if, at the beginning of 1944, six months before D-Day, Gen. George C. Marshall, the Army chief of staff, told Gen. Dwight D. Eisenhower, the supreme allied commander, that it was time to give someone else a chance to lead.”

Petraeus, a four-star general, took over the Afghan command in July 2010 to fill a void after Gen. Stanley A. McChrystal was fired because of scathing remarks about America’s civilian leadership. McChrystal’s predecessor, Gen. David McKiernan, was ousted on May 11, 2009, a year before his term as commander was set to end because newly elected President Barack Obama wanted a new war policy. He had succeeded Gen. Dan McNeill, who served in 2007-08.

Petraeus completed a one-year term and retired to become CIA director in September 2011.

He resigned Nov. 9 after he had an extramarital affair with his biographer. Allen, who also has four stars, is under investigation following revelations that he exchanged thousands of emails with a Florida socialite also involved in the Petraeus case, including a few which were found to be of a questionable nature.

Some analysts and former military officers say that rotating generals so quickly creates a disconnect between the commanders and their Afghan allies, including the mercurial President Hamid Karzai. Commanders also have to deal with billions of dollars in funds and the complexities of handing over security to the Afghans by the end of 2014, including building an army and police of 352,000 almost from scratch.

Gen. Mohammad Zahir Azimi, a spokesman for the Afghan Ministry of Defense, said the longer a NATO commander stays in the job, the more chance he has to really understand Afghanistan.

“For us, an Afghan army corps commander should stay in his position for at least three years,” he said.

Jawed Kohistani, military analyst in Kabul, said he thinks a constant changeover of senior NATO commanders or Afghan military leaders hampers coordination of the two forces. Staying longer, he believes, allows a commander to know insurgents and their weaknesses.

“It gives an opportunity for the enemy to use this gap — the time between the leaving of one commander and the arrival of another — to their advantage,” Kohistani said. “There should be enough time for a NATO commander to get to know the Afghan president, vice presidents, security ministers and assess the situation. If he doesn’t have enough time to do all these things, it has a negative effect on the security situation.”

Obama sped up the confirmation hearing last week for Gen. Joseph Dunford to become the 15th ISAF commander and replace Allen, who was to leave in the spring after 18 months at the helm for a new job as U.S. European Command chief and NATO supreme allied commander. But Allen’s confirmation has been postponed until an investigation into his role is concluded.

There have also been about a half-dozen U.S. generals who only commanded American combat troops in the first years of the conflict.

By comparison, the command tours of generals in Iraq “averaged almost twice as long as ISAF’s,” said Stephen Biddle, a professor of political science and international affairs at George Washington University.

Given the huge funds involved in the war effort, running the Afghan campaign has been as complicated as managing a multibillion dollar corporation.

Anthony Cordesman, a national security expert at the Center for Strategic and International Studies, said the lack of continuity includes both the military and civilian presence in Afghanistan, including the State Department and other agencies.

“What the Afghans see is constant change at every level,” he said. “They constantly see people come and go. They have no reason to establish lasting relationships. People leave at the point where they’re becoming most effective.”

He added that “this constant rotation is a problem everybody recognizes, but no one has really been willing to address.”

The Pentagon also has a number of senior leaders — ranging from the chairman of the joint chiefs of staff and defense secretary to the commander of U.S. Central Command — who also play key roles in the war strategy and provide some continuity.

“There is no doubt that the frequent changeover is tough. There is a learning curve each time a new man takes the helm,”

“The good aspect to this is that it brings a fresh set of eyes,” said Michael O’Hanlon, a fellow at the Brookings Institution in Washington. “It’s not realistic to have commanders serve a whole lot longer than, say, Gen. Allen … as these folks get tired.”