IN CAMP LEATHERNECK, Afghanistan — In the late 1950s, scores of U.S. engineers transformed a swath of uninhabited desert in southern Afghanistan into verdant farmland by constructing a network of irrigation canals fed by the Helmand River. The Afghan government filled the area, which it called Marja, with Pashtun nomads and told them to grow wheat.
The wheat fields have since been replaced by tracts of opium-producing poppies. The mud-walled compounds that once housed families now conceal drug-processing labs and roadside-bomb factories. And the canals serve as moats to protect hundreds of Taliban fighters, who use Marja as a staging area for attacks across Helmand province.
In the coming days, thousands of U.S. Marines will seek to transform Marja once again. Working with Afghan soldiers, the Marines are planning a major operation to flush out insurgents and allow the Afghan government to reassert control.
On Thursday, U.S. and Afghan forces ringed the Taliban stronghold, sealing off escape routes and setting the stage for what is being described as the biggest offensive of the nine-year war.
"We intend to go in big, strong and fast," said Brig. Gen. Larry Nicholson, commander of the 2nd Marine Expeditionary Brigade.
British forces plan to conduct simultaneous operations intended to push into other Taliban strongholds in Helmand. The combined operations are expected to involve about 15,000 U.S., British and Afghan forces, NATO military officials said.
Nicholson said he anticipates a tough fight. Not only do the canals pose a significant logistical challenge for moving troops into the area — the waterways are too wide and deep to drive through — insurgents also have planted numerous homemade bombs along the approaches.
There are so many insurgents and roadside bombs in Marja that the Marines have not entered the area since arriving in Helmand last summer.
Soon after arriving in the province, Marine officers told tribal elders that an invasion of Marja was inevitable, part of an effort to persuade low-level Taliban fighters to reconcile with the government.
Afghan leaders met Thursday with tribal elders to tell them that, indeed, government and NATO forces would soon rid the area of Taliban fighters and that Afghan police and soldiers would stay behind after the fighting was over.
Hanif Atmar, the Afghan interior minister, told a group of about 350 elders from Marja's main tribes that a major military operation would begin in the area soon. When it was over, he said, about 1,000 police officers would be assigned to help keep the Taliban out. Atmar also promised that the government would initiate a number of development projects, including roads and health clinics.
And while Marja's leaders said they had no love for the Taliban, many expressed deep skepticism that Afghanistan's leaders would make good on their promises to protect them from insurgents.
"Yes, we want this operation in our area — but do not leave, as you have in other areas, and let the Taliban come back," Haji Abdul Rashid, one of the elders, told Atmar.
The success of this operation depends as much on what happens after the battle as during it. Coalition leaders want to install a stable civilian government as soon as they manage to dislodge the Taliban. They hope that by creating a secure environment — and doling out liberal amounts of aid — they can persuade locals to abandon their support of the Taliban and give farmers a viable alternative to growing opium poppies.
That last goal might be the most vexing. Opium has been extremely lucrative for local power barons who split their profits with the Taliban. Some experts predict that anyone who switches sides, or even appears to be cooperating with the Afghan government, would be targeted for assassination.
Marine intelligence officers estimate that several hundred fighters are in the area. Many are local residents who could switch allegiances under pressure, but dozens are hardened insurgents.
U.S. and NATO commanders contend that telling Afghans that the operation is imminent also could help prevent Afghan President Hamid Karzai from backing down in the face of pressure from tribal chieftains who have profited from Marja's drug industry. For now, however, "the Afghan government is fully behind this operation," said British Maj. Gen. Nick Carter, the top allied commander in southern Afghanistan.
The push into Marja will continue a Marine effort to mount counterinsurgency operations along the Helmand River valley. The region, home to about 750,000 people, is of particular concern to military commanders because it serves as an infiltration route for fighters coming from Pakistan and because it is where much of the country's poppy is grown. Military officials regard pacifying the valley as essential to reversing Taliban gains in and around Kandahar, the country's second-largest city.
A Marine operation in July to wrest control of key towns along the river has produced encouraging results — people who had fled are returning home, shops have reopened and schools are operating again — but military officials and local leaders said the gains have remained fragile. The Taliban has used its redoubt in Marja to mount attacks on Marine units and manufacture bombs.
Once Marja is secure, the State Department and the U.S. Agency for International Development plan to assist farmers in planting crops and rehabilitating the canal network. An unstated aim is to salvage a project the United States began almost 60 years ago.