On the morning of the sweep, made by Weapons Company, Third Battalion, First Marines, a large communications antenna that rose from one compound vanished before the Marines could reach it. The man inside insisted that he had seen nothing. And when the Marines moved within the compounds’ walls, people in nearby houses released white pigeons, revealing the Americans’ locations to anyone watching from afar.

The Taliban and their supporters use other signals besides car horns and pigeons, including kites flown near American movements and dense puffs of smoke released from chimneys near where a unit patrols.

“You’ll go to one place, and for some reason there will be a big plume of smoke ahead of you,” said Capt. Paul D. Stubbs, the Weapons Company commander. “As you go to the next place, there will be another.”

“Our impression,” he added, “is the people are doing it because they are getting paid to do it.”

Late in the morning during the company’s sweep, the insurgents fired a few bursts of automatic rifle fire from outside the cordon. Later still, they lobbed a single mortar round toward the company. It exploded in a field without causing any harm.

No one could tell exactly where the fire came from. This showed another side of the Taliban’s local activities. Wary of engaging the Marines while they were ready and massed, fighters risked nothing more than this harassing fire.

The sweep was not entirely fruitless. In several houses, Afghan soldiers found sacks of poppy seeds, which they carried outside, slashed open with knives and set on fire. In a few houses, they found processed opium and heroin. But the Taliban’s fighters had proved elusive again.