GARMSER, Afghanistan — The Marines of Bravo Company’s 1st Platoon sleep beside a grove of poppies. Troops in the 2nd Platoon playfully swat at the heavy opium bulbs while walking through the fields. Afghan laborers scraping the plant’s gooey resin smile and wave.

Last week, the 24th Marine Expeditionary Unit moved into southern Helmand province, the world’s largest opium poppy-growing region, and now find themselves surrounded by green fields of the illegal plants that produce the main ingredient of heroin.

The Taliban, whose fighters are exchanging daily fire with the Marines in Garmser, derives up to $100 million a year from the poppy harvest by taxing farmers and charging safe passage fees — money that will buy weapons for use against America, NATO, and Afghan troops.

Yet the Marines are not destroying the plants. In fact, they are reassuring villagers the poppies won’t be touched. American commanders say the Marines would only alienate people and drive them to take up arms if they eliminated the impoverished Afghans’ only source of income.

Many Marines in the field are scratching their heads over the situation.

“It’s kind of weird. We’re coming over here to fight the Taliban. We see this. We know it’s bad. But at the same time we know it’s the only way locals can make money,” 1st Lieutenant Adam Lynch, 27, of Barnstable, Mass, said.

The Marines’ battalion commander, Lieutenant Colonel Anthony Henderson, said in an interview yesterday that the poppy crop “will come and go” and that his troops can’t focus on it when Taliban fighters around Garmser are “terrorizing the people.”

“I think by focusing on the Taliban, the poppies will go away,” Colonel Henderson, a 41-year-old from Washington, D.C, said. He said once the militant fighters are forced out, the Afghan government can move in and offer alternatives.