COMBAT OUTPOST PAYNE, Afghanistan – And just like that, we’ve reached the Marine Corps’ southern frontier in Helmand province. Photographer Colin Kelly and I traveled by MV-22 Osprey today to this desert outpost in Khanashin district, where most people live in a collection of rural villages along a bend in the Helmand River. It was first flooded with Marine forces in July 2009 during Operation Khanjar, or Strike the Sword, becoming part of the biggest aerial assault since the Vietnam War. COP Payne was established afterward and served as the battalion headquarters for the last few years for light armored…