CAMP SHORAB, Afghanistan — The last time American Marines were in Helmand Province, they had a sweeping mission and the resources to carry it out.

Tens of thousands of Marines flooded the southern villages here, and they had tens of millions of dollars in cash to throw into projects that would bolster local support. They spearheaded some of the bloodiest battles of this long war, losing 349 dead in the process, but they succeeded in turning much of what had been prime Taliban and opium territory over to the Afghan government.

Then, following the Obama administration’s deadline to have combat forces out of Afghanistan by the end of 2014, they went home.

Now they are back in Helmand with a changed mission, looking at an overwhelmingly changed map.

Within two years, Helmand, Afganistan’s largest province, all but returned to the hands of the Taliban. Lashkar Gah, the provincial capital and a haven for tens of thousands of refugees from the fighting, has been surrounded for more than a year, and in that time has been infiltrated by insurgents at least three times. The performance of the Afghan forces, left on their own, was catastrophic, with 60 to 70 soldiers dying each week.