Nawa, Afghanistan ? U.S. Marines suffered their first casualties of a massive new military campaign Thursday as they engaged in sporadic gunbattles along 55 miles of Taliban-controlled heartland in southern Afghanistan.

One Marine was killed and several others were injured or wounded on the first full day of the assault, the largest military operation in Afghanistan since the fall of Taliban government in 2001.

The offensive will test the Obama administration’s new strategy of holding territory and letting the Afghan government sink roots in Helmand province. The insurgency has proven particularly resilient in this area, where foreign troops have never before operated in such large numbers.

President Barack Obama told The Associated Press in an interview Thursday that he has a “very narrow definition of success when it comes to our national security interests” in the region. “And that is that al-Qaida and its affiliates cannot set up safe havens from which to attack Americans.”

“I think we can measure it by whether or not they’ve got training camps where people are coming in and getting trained in explosives, being sent out and directed in carrying out terrorist activity,” Obama said in Washington.

An immediate goal, the military says, is to clear away insurgents before the nation’s Aug. 20 presidential election. Southern Afghanistan is a Taliban stronghold but also a region where Afghan President Hamid Karzai is seeking votes from fellow Pashtun tribesmen. Without such a large Marine assault, the Afghan government would likely not be able to set up voting booths to which citizens could safely travel.

The Pentagon is deploying 21,000 additional troops to Afghanistan in time for the elections and expects the total number of U.S. forces there to reach 68,000 by year’s end. That is double the number of troops in Afghanistan in 2008 but still half as many as are now in Iraq.

Even bigger challenges, perhaps, will come in the weeks and months after the Marines have established their presence here.

The U.S. will have an opportunity to help develop alternate livelihoods for farmers whose opium poppy crops bankroll the Taliban. Helmand province is the world’s largest opium poppy-producing area.

Obama told the AP he wants to help ensure that Afghans “are benefiting from development and improved agricultural systems and education systems and health care systems.”

He also said Washington and its allies must build up the Afghan national army and police and help Afghanistan and neighboring Pakistan secure their common border.

“The benchmarks of success that we’ve laid out are: Are we building an Afghan national army and police structure that can secure itself without the assistance of NATO forces or U.S. forces? Is Pakistan able to maintain its borders so that al-Qaida or affiliates aren’t operating there?” Obama said.

Pakistan’s army said Thursday 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. Helmand’s strategic setting will give the U.S. an opportunity to interdict fighters coming from Pakistan

Elsewhere, the U.S. military announced that insurgents were believed to have captured an American soldier in eastern Afghanistan on Tuesday. The missing soldier was not involved in Operation Khanjar, or “Strike of the Sword,” under way in southern Afghanistan.

The southern offensive was launched shortly after 1 a.m. Thursday as thousands of Marines poured from helicopters and armored vehicles into villages along the Helmand River. Officials described the offensive, involving almost 4,000 newly arrived Marines and more than 600 Afghan security forces, as the largest and fastest-moving of the war’s new phase.

The troops fanned out into the districts of Nawa and Garmser in central Helmand and up to 55 miles south in the vicinity of Khan Neshin, the capital of Rig district, according to the military.

Last summer, the 24th Marine Expeditionary Unit took the Garmser town and helped provide security for an area U.S. commanders say is now relatively secure. The U.S. would now like to replicate that success elsewhere in the province.

In Nawa village, Marines took militants by surprise by dropping behind Taliban lines, said Capt. Drew Schoenmaker, 31, of Greene, N.Y.

“We are kind of forging new ground here. We are going to a place nobody has been before,” said Schoenmaker, who commands Bravo Company of the 1st Battalion, 5th Marine Regiment.