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The wheat fields have since been replaced by tracts of opium-producing poppies.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 in partnership with Afghan soldiers, the Marines are planning a major operation to flush out insurgents and allow the Afghan government to reassert control.

"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.

U.S. forces have moved into positions around Marja over the past week in preparation. About 200 Marines and Afghan soldiers, traveling by helicopters, seized a key intersection northeast of Marja on Tuesday morning, 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. Speaking to his troops Tuesday, Nicholson called Marja "the last spot where the enemy feels secure" in the Marines' area of operations in Helmand.

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. Senior U.S. and NATO commanders and Afghan military officials have publicly echoed that message in recent days.

It is not clear how effective the threat will be. 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.

Key insurgent leaders and their lieutenants also may repeat a tactic they have employed in the face of previous U.S. offensives: plant bombs and flee to places with few security forces.

U.S. and NATO commanders contend that telling Afghans that the operation is imminent also could help prevent Afghan President Hamid Karzai, who gave his approval for the mission two weeks ago, 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.

Afghan leaders and foreign military commanders differ over how civilians should respond to the offensive. Interior Minister Hanif Atmar told senior U.S. and NATO military officials that civilians should be encouraged to flee the area. But Carter said U.S. and British commanders want Marja residents to stay put.

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.

One key difference between the operation last summer and the upcoming mission is the involvement of Afghan army and police units. Only a few hundred Afghan soldiers had accompanied the Marine units in July. For this invasion, military officials said, each Marine battalion will be partnered with an Afghan one. Several hundred Afghan paramilitary police officers also are ready to be deployed in Marja once areas are cleared of insurgents, the officials said.

The U.S. government has designated a civilian reconstruction team to move into Marja when the fighting subsides. An American team also is working with the Karzai government to deploy a contingent of Afghan civil servants. To encourage Afghans to serve in Marja, the government plans to increase the average monthly salary for such personnel from $60 to about $300.

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.

After the canals were built, the Afghan government began moving Pashtun nomads to Marja in 1959. Authorities hoped settling them would keep them from allying with Pashtuns in neighboring Pakistan who were agitating for an independent homeland.

The settlers received almost 15 acres of land, two oxen and free seeds. The government, with U.S. assistance, set up 11 schools in the area. American teachers, funded by USAID, staffed some of the schools in the 1960s and 1970s.

But the development experiment didn't work. The engineers failed to account for an impermeable crust several feet below ground, which hindered drainage. Because the former nomads were not irrigation farmers, they exacerbated the problem by flooding their fields with too much water.

"From the beginning, the project was plagued with basic cross-cultural misunderstandings and technical miscalculations," a USAID hydrologist wrote in a 1973 analysis.

The solution proposed by USAID development specialists was to move everyone off the land, level the area with bulldozers and then return the farmers to their plots. But the farmers refused to leave, and they met the bulldozers with rifles, forcing the project to be scrapped.