US Marines and Afghan forces launched an attack on the Taliban-held city of Marjah in Helmand, Afghanistan this Saturday morning. The operation is the largest of its kind since President Barack Obama announced 30,000 American reinforcements for the war last December.

Between 400 and 1,000 insurgents are estimated to have gathered in Marjah. The city, with a population of 80,000, is the largest under Taliban control in the south of Afghanistan and the hub of their logistical and opium-smuggling network.

Several hundred US Marines and some Afghan soldiers were among the first wave of troops, flown in by helicopter. The offensive involves close combat in difficult terrain. A close grid of wide canals dug by the United States as an aid project decades ago make the territory a rich agricultural prize, but complicate the advance of American forces.

On the eve of the attack Friday, cars and trucks jammed the main road out of the city as hundreds of civilians fled the area ahead of the assault. For weeks, American commanders have signaled their intention to attack Marjah, hoping that civilians would seek shelter.

The operation, codenamed “Moshtarak,” or Together, is described as the largest joint offensive of the Afghan war. 15,000 troops are involved, including some 7,500 fighting in Marjah and British forces to the north in the district of Nad Ali.

Once the town is secured, NATO hopes to rush in aid and restore public services in a bid to win support among the population. The Afghans’ ability to restore those services is crucial to the success of the operation and to the preventing of the return of the Taliban.