They hope to retake the 10 percent of Afghanistan the Taliban holds.
Some of the Marines that make up the 24th Marine Expeditionary Unit helped to tame a thriving insurgency in western Iraq. The newly arrived forces hope to move into regions of Afghanistan now controlled by the Taliban.
The troops are working alongside British forces in Helmand province - the world's largest opium-poppy region and site of the fiercest Taliban resistance over the last two years. The director of U.S. intelligence has said the Taliban controls 10 percent of Afghanistan - much of that in Helmand.
"Our mission is to come here and essentially set the conditions, make Afghanistan a better place, provide some security, allow for the expansion of governance in those same areas," said Col. Peter Petronzio, the unit's commander.
Thirteen of the 19 Marines in the platoon of 1st Lt. Adam Lynch, 27, served in 2006 and 2007 in Ramadi, the capital of the Anbar province in western Iraq. The vast region was once al Qaeda in Iraq's stronghold before the militants were pushed out in early 2007.
Lynch expects the Marines, who arrived last month on a seven-month deployment, will help calm Helmand as well.
"If you flood a city with Marines, it's going to quiet down," Lynch said in between sets of push-ups on Helmand province's sandy ground. "We know for seven months we're not here to occupy, we're just here to set conditions for whoever comes in after us."
Taliban fighters have largely shunned head-on battles since losing hundreds of fighters in the Panjwayi region of Kandahar province in fall 2006, and it's not clear that Taliban fighters will stay to face the Marines in regions they operate.
Lynch, a mobile assault commander, said he doesn't care if the militants flee: "Just get the Taliban out of here, that's the biggest thing."
Western countries, including the U.S. and other NATO nations, have been sending more troops to Afghanistan as violence has escalated.
More than 8,000 people, mainly militants, were killed in insurgency-related violence in 2007, the U.N. says.
The number of suicide attacks spiked in 2007, with the Taliban launching more than 140 suicide missions, the highest number since 2001 invasion to oust the Taliban for hosting al Qaeda leader Osama bin Laden.
The U.S. now has 32,500 troops in the country - the most since the 2001 invasion. In late 2006, Afghanistan had 40,000 international troops. Today, that number is almost 70,000.
But Western officials have warned in recent months that the international mission could fail. Washington has lobbied for NATO nations to provide more troops in Afghanistan, and in particular to add forces in the southern and eastern areas which have seen most of the recent fighting against the resurgent Taliban.
The Marines' presence in southern Afghanistan is a clear sign that neither Britain nor Canada - which operates in nearby Kandahar province - have enough troops to control the region. But commanders and troops say the countries are working well together.
British Capt. Alex West helped deliver supplies to a remote and dusty firebase in Helmand province about a week ago.
"We spent the last operations borrowing kit (gear) off you, so it's about time you borrow stuff from us," said West, 29, of Colchester, England. "All of us have been in operations where the American have helped us, so we're happy to help."
The Marines are known as the theater task force, meaning they fall under the direct control of U.S. Gen. Dan McNeill, the commander of NATO troops in Afghanistan. McNeill can move the Marines to whatever flashpoint he wants. Most other U.S. troops are stationed at permanent bases in the east.
The Marines have been moving supplies and forces through Helmand by ground convoys the last several weeks, a draining and dangerous task. Some convoys have taken more than 20 hours to complete, and two Marines were killed by a roadside bomb April 15.
Lt. Col. Ricky Brown, the commander of the logistics battalion, gave a pep talk to a supply convoy last week, hinting at operations to come.
"You all are gonna move down there so the BLT (battalion landing team) can go in there and kick some Taliban butt," he said.
They have also been given directions to steer clear of the region's poppy fields so they don't risk alienating local farmers who rely on the cash crop for their income.
Counter-insurgency doctrine calls for forces to first clear a region of militants, hold that region and then build up government institutions and businesses. But the Marines are in the country for only seven months, meaning they don't have time to hold and build regions. But it's not clear if there are enough other NATO troops to hold areas, either.
"We are the clear piece," said Clinton. "There are others who will do the holding and building. We're clearing and doing some holding."
While riding in a 47-vehicle convoy through the sands of Helmand province this past week, 1st Lt. Dan Brown said the terrain reminded him of other missions.
"If you didn't know any better you'd think you were in Anbar right now," he said, referring to western Iraq.