One Marine was killed and several others were injured or wounded throughout the day, the military announced.
Officials described the offensive as the largest and fastest-moving of the war's new phase and the biggest Marine assault since the one in Fallujah, Iraq, in 2004. It involves nearly 4,000 newly arrived Marines plus 650 Afghan forces. British forces last week led similar, but smaller, missions to clear out insurgents in Helmand and neighboring Kandahar province.
"Where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces," Marine Corps Brig. Gen. Larry Nicholson said in a statement.
Pakistan's army said it had moved troops from elsewhere on its side of the Afghan border to the stretch opposite Helmand to try to stop any militants from fleeing the offensive. It gave no more details, but U.S. and Pakistani officials have expressed concern that stepped-up operations in southern Afghanistan could push the insurgents across the border.
Transport helicopters carried hundreds of Marines into the village of Nawa, some 20 miles south of the provincial capital of Lashkar Gah, in a region where no U.S. or other NATO troops have operated in large numbers.
The troops took many insurgents by surprise, dropping behind Taliban lines, said Capt. Drew Schoenmaker, from Greene, N.Y.
"We are kind of forging new ground here. We are going to a place nobody has been before," said Schoenmaker, 31, who commands Bravo Company of the 1st Battalion, 5th Marine Regiment.
Several hundred Marines took positions in a freshly plowed dirt field at 3 a.m. The soft, deep dirt proved challenging for troops weighed down with days' worth of water, food and gear, and many frequently stumbled.
At daybreak the Marines walked along tree lines, and at 6:15 a.m. the company took its first incoming fire, likely from an AK-47 along a tree-line. The next three hours brought repeated bursts of gunfire and volleys of rocket-propelled grenades, sending deep booms across the countryside.
A small force of Afghan soldiers accompanying the Camp Pendleton-based Marines got into several scraps with an insurgent force of about 20 fighters. The fire came from a mud-brick compound, and the Marines, the Afghan soldiers and their British advisers surrounded the compound on the east and the south.
Before the mission, Schoenmaker, the company commander, said he would practice "tactical patience" as a way to avoid civilian casualties -- an issue newly arrived Gen. Stanley McChrystal has underscored in recent weeks. Though troops in many similar circumstances have called in airstrikes on such a militant-controlled compound, Schoenmaker did not.
"We made the decision to isolate the compound and not destroy it because we couldn't confirm if civilians were inside," he said. The militants were believed to have escaped out the back.
A Cobra helicopter circling overhead for most of the day fired rockets at a tree line nearby. Other troops walked through fields of corn and past mud-wall homes. Only a handful of villagers dared to venture outside.
Helmand's deadly heat, well over 100 degrees Fahrenheit, proved to be another enemy the Marines had to fight. Because soldiers were on foot, they had to carry all their own water and food. Forward observers and snipers spent the entire day under the cloudless sky.
"It's like when you open up the oven when you're cooking a pizza and you want to see if it's done. You get that blast of hot air. That's how it feels the whole time," said Lance Corp. Charlie Duggan Jr., 21, of Baldwinsville, N.Y.
The Marines trained for months in the heat of the Mojave desert for the deployment, and many appeared happy to be here.