FORWARD OPERATING BASE WHITE HOUSE, Afghanistan – We’ve reached the final frontier in Helmand province. This base serves as headquarters to 1st Battalion, 8th Marines, which oversees combat operations in Kajaki district. Top Marine officers in Afghanistan acknowledge they’ve pushed as far north here as possible ahead of the planned drawdown, and will seek this summer to solidify gains already made, rather than pushing farther into the countryside. There were about 20,000 Marines in theater last summer, but that number could drop to as low as 7,000 this fall. In coming weeks, we’ll have several long-form pieces assessing the future…