WASHINGTON (AP) - From tasteless photos to urinating on dead insurgents, bad behavior by U.S. troops in Afghanistan has hampered America's war effort over the past year, triggering a broad new campaign by defense leaders to improve discipline in the ranks.

Defense Secretary Leon Panetta, in his first personal appeal to troops on the issue, is expected Friday to remind U.S. forces that they are representing the American people and they must behave up to military standards.

Panetta will speak to soldiers at Fort Benning, Georgia, and he is expected to urge them to act as leaders and look after their comrades. His remarks are expected to reflect recent talks by the Army and Marine Corps chiefs telling their commanders to get their troops in line.

The service leaders have zeroed in on discipline in meetings with mid-level commanders around the country. They say they recognize that part of the problem may be leadership stumbles by the young officers who have shouldered much of the burden of wars in Iraq and Afghanistan.

"Maybe we've gotten overconfident and maybe we've gotten a little bit comfortable in our young leaders," Gen. Ray Odierno, the Army chief of staff, told The Associated Press in an interview Thursday. "Realizing that they are young, they don't have a lot of experiences. We have to continue to assist them so they understand what is expected of them."

Marine Corps Commandant James Amos was blunter.

"We are allowing our standards to erode," he wrote his commanders. "A number of recent widely publicized incidents have brought discredit on the Marine Corps and reverberated at the strategic level. The undisciplined conduct represented in these incidents threatens to overshadow all our good work and sacrifice."

Senior leaders have warned for several years about a deterioration of discipline that may have contributed to increased substance abuse, suicides, domestic abuse and other problems.

In January, Marines were found to have made a video showing them urinating on Afghan insurgents' corpses. In February, troops mistakenly burned copies of the Quran, which led to violent protests and revenge killings of six Americans. In March, a U.S. soldier left his base and allegedly killed 17 civilians, mainly women and children. Last month, newly revealed photographs showed U.S. soldiers posing in 2010 with Afghan police holding the severed legs of a suicide bomber.

Marine Gen. John Allen, the top U.S. commander in Afghanistan, has expressed concern about the impact that those incidents have had on the war, according to a senior defense official. Allen believes that a number of major setbacks in the past six months have resulted from moral, not operational, failures, said the official, who spoke on condition of anonymity to discuss internal assessments.

Insurgents have used the incidents to incite violence and undermine U.S. efforts to win over the Afghan people, considered critical to counterterrorism operations. The incidents have reinforced the perception of Americans as unfriendly or occupying forces who do not understand the culture or the religion of the people they are supposed to protect.

Such ethical lapses have occurred in war through the centuries. But military officials and outside experts generally agree that America's longest war has put unprecedented strain on the country's all-volunteer military, an overwhelmingly young force that needs supervision and strong leadership.

In earlier conflicts such as those in Vietnam or Korea, such incidents were not as readily visible. Today, they end up on YouTube in seconds, viewed by an audience that does not always attribute such behavior to the stress of war.

After writing his letter to Marine commanders, Amos began taking his message to bases and stations in talks with officers. And Odierno included the topic during meetings with his two- and three-star commanders, as well as in talks with younger officers he sees during base visits.

Odierno said that overall the force has behaved admirably over the past 11 years of war and that troops understand the importance of standards and discipline.

"We're putting a lot more responsibility on very young leaders, lieutenants and sergeants," Odierno said. "We just have to remind everybody that we have to put the checks and balances in place, and we have to remind everybody about the importance of culture and the profession."