WASHINGTON (CNN) -- As Marines mount a new U.S. push to force Taliban fighters from a stronghold in southern Afghanistan, a new military strategy is taking root.
That strategy might best be described as: After taking a town from the enemy, hold that town. And after pushing the enemy into the hills, keep him there.
The strategy also includes persuading local residents to feel more secure and then cutting the Taliban's financial lifeline by convincing Afghan farmers to grow an increasingly lucrative wheat crop instead of poppies, which would be used to produce heroin.
It's a strategy many U.S. military leaders have wanted to pursue for years. Until recently, however, they didn't have sufficient troop levels to pull it off.
"They're not just doing an offensive push to get bad guys," said a senior defense official, who requested anonymity because he's not authorized to speak publicly about U.S. war strategy. "They're going in to hold the area and stay there."
"This approach is indicative of [commanding U.S. Gen. Stanley] McChrystal's philosophy: measuring success by the number of Afghans protected, not bad guys killed."
The shift is not without risk or cost. U.S. troops are interacting much more closely with local villagers in an effort to win their trust and relying less on distant airstrikes that may result in civilian casualties.
On Thursday, one Marine was killed and several others were wounded in the U.S.-led offensive in the southern Helmand Province. Some also suffered heat-related illnesses.
June was the deadliest month in Afghanistan for U.S. troops since September. The Pentagon says 25 U.S. troops and 38 coalition troops died in June.
But McChrystal, who took over as the allied commander in Afghanistan last month, told senators during his confirmation hearing that the nearly eight-year war in the struggling Islamic republic requires a new focus on counterinsurgency to reduce violence and build local support.
"Although I expect stiff fighting ahead, the measure of success will not be [enemies] killed. It will be shielding the Afghan population from violence," he said.
Roughly 4,000 U.S. Marines and sailors, joined by British troops and several hundred Afghan security forces, are participating in Operation Khanjar, the drive to secure Helmand Province in advance of Afghanistan's August presidential elections.
Helmand was once known as the breadbasket of Afghanistan, but the fertile land is now used for growing poppies. Afghanistan produces more than 90 percent of the world's opium, with most of that coming from the poppies in Helmand.
The drug trade is an important source of income for the Taliban, and major supply routes run through the province.
Cut that trade, many observers believe, and you can break the Taliban's financial back.
The military shift is part of President Obama's new strategy toward Afghanistan. Obama has taken a political risk by committing 21,000 additional troops to the country, which was the original front in U.S. military action after the September 11 attacks.
Obama promised, in announcing the change, that American soldiers and Marines "will take the fight to the Taliban in the south and the east, and give us a greater capacity to partner with Afghan security forces and to go after insurgents along the border."
The Islamic fundamentalist Taliban ruled most of Afghanistan before its allies in the al Qaeda terrorist network attacked New York and Washington on September 11, 2001.
Though they were quickly toppled after the attacks, its leaders escaped, and the movement regrouped in the Afghan countryside and across the border in Pakistan.
CNN's Atia Abawi and Barbara Starr contributed to this report.
All About Afghanistan War • Stanley McChrystal • Barack Obama