Morale among American troops underwent a “significant decline” between 2005 and 2009, according to a study by the US military released Thursday.

Soldiers also experienced “significantly higher” instances of “acute stress,” including depression or anxiety, during the same time period.

Nearly half of combat troops surveyed say that they had killed an enemy fighter.

The findings come after more than 1,200 soldiers and Marines in Iraq and Afghanistan completed surveys between July and August of last year. Now, the study is being released as America’s longest war – in Afghanistan – is about to enter its second decade.

Senior US military officials say they are hopeful that the research will provide insights into better caring for American soldiers currently facing “incredibly high” levels of combat.

The increased exposure to heavy fighting appears to be the No. 1 reason for the decrease in morale among soldiers, according to US military officials. “As a group, we were struck by the fact that levels of combat are extremely high,” says Col. Paul Bliese, director of the division of psychiatry and neuroscience at the Walter Reed Army Institute of Research in Washington.

Nearly two-thirds of the respondents in the survey, for example, reported having roadside bombs explode “near” them, and more than three-quarters of troops surveyed say that they had seen a fellow soldier in their unit killed.

Some 80 percent reported “shooting at [the] enemy,” and nearly half, 48 percent, said they were “responsible for the death of [a] combatant.”

War takes a psychological toll, say officials, who add that the more tours a soldier has served, the more likely he or she is to experience aftereffects.

“There are few stresses on the human psyche as extreme as exposure to combat,” says Lt. Gen. Eric Schoomaker, the Army’s surgeon general and head of the Army’s medical command.

How to best treat active-duty soldiers experiencing such stress is the Pentagon’s ongoing quest. One thing the military has been doing is to keep track of rates of reported use of alcohol and medications, which troops often say they use “to mask some of the symptoms of the problems they’re having,” says Schoomaker.

At the same time, the study found, troops are increasingly comfortable seeking help when they feel they need it. Moreover, troops reported feeling better trained and better equipped for the stresses of combat.

Factors such as realistic training, unit cohesion, and a clear sense of mission appear to mitigate the effects of stress, says Capt. Frederick Kass, health services deputy for the US Navy Headquarters Marine Corps.

While at war, treatments for stress may be quite straightforward, military official add. As a result of the survey, for example, the Pentagon is investigating how to encourage troops to simply get more sleep. “We’ve all been concerned ... about the pervasive nature of sleep deprivation,” Schoomaker says.

This is a particular challenge since troops tend to get “well less than seven hours” of sleep a night, says Bliese of the Walter Reed Army Institute of Research. Then there are the night operations that are a staple of combat. All this tends to have a “chronic impact on the cognitive ability” of troops, he adds.

Troops today are grappling with some of the same issues that soldiers from America’s previous wars took on, when “post-traumatic stress” was known as shell shock or battle fatigue, military official say. “I came out of this with an increased respect for what warriors in past conflicts have endured,” Schoomaker says – “and endured sometimes quietly.”